chore(#68): rotate signing keys, key-rotation re-attestation mode, harden show-seed-b64

Wires the maintainer's freshly-rotated signing keys into BCC (issue #68
finding 5: the old key was shared between catalog+release and had been
exposed to CI), adds a key-rotation re-attestation mode to the Catalog
Console so the Console can actually re-sign under the new key, and
hardens the show-seed-b64 CLI prompt that led to a private key being
pasted into a chat.

## New keys (#68 finding 5)

- bcc_core.CATALOG_PUBKEYS -> new CATALOG pubkey
  (0s24PmkZcTT5yxNDdyTPHl5fyxArrHNPJKBjnXoQd8k=). Signs data/catalog.json,
  Console-only, offline.
- .github/workflows/ci.yml EXPECTED_CATALOG_PUBKEY_B64 -> same new
  catalog pubkey (the CI trust anchor added for #68 finding 4).
- scripts/sign_checksums.py RELEASE_PUBKEYS -> new RELEASE pubkey
  (6BnPgJEHJFyVltFoLTCNadIsehjy00iiW8IRlC1TfhA=). Signs SHA256SUMS only,
  CI-resident.
- README.md 'Verifying your download' / 'Signing keys' sections filled in
  with both pubkeys, explicit about which key is which.

The old key 082NOwVB7uURkvfyS3+knJ+40Fk6C9unsF47+2uPKo4= is retired (it
was shared and exposed to CI) and is deliberately NOT retained in either
trust list -- keeping a burned key in CATALOG_PUBKEYS would defeat the
point of rotating it.

## Key-rotation re-attestation mode (Catalog Console)

Problem: after the key swap, the existing data/catalog.json.sig (signed
with the OLD key) no longer verifies under the NEW CATALOG_PUBKEYS, but
catalog content is unchanged, so diff_catalogs(last_signed, current) is
empty -- and can_sign()'s empty-diff guard (load-bearing, #68 finding 1)
correctly refuses to sign an empty changeset. Without a rotation-aware
path, the Console could never re-sign and CI would stay red forever.

Fix: treat rotation as a full re-attestation, not a diff.

- catalog_review.py: ReviewSession/start_review gain reattest: bool =
  False. When set, changes is built via diff_catalogs(None, new_catalog)
  -- every entry presented as if newly added, requiring a fresh
  acknowledgement -- instead of diffing against old_catalog. can_sign()
  is UNCHANGED: it still refuses a genuinely-empty changeset and still
  enforces the TOCTOU blob-SHA pin and the blocking-risk check, because
  reattest sessions simply never produce an empty changeset (unless the
  catalog itself is empty).
- catalog_console.py: adds catalog_signature_valid_at(repo_dir, commit,
  raw), which calls bcc_core.verify_catalog_signature directly (never
  reimplemented) to detect whether the committed .sig verifies under the
  CURRENT CATALOG_PUBKEYS. ReviewWindow._on_load's source=main path uses
  this to decide reattest=True/False, and shows a loud, explicit red
  banner ("KEY ROTATION IN PROGRESS...") whenever reattest mode is
  entered -- never silent. Status text and Sign-button gating flow
  through the same can_sign()/all_entries_acknowledged() path as normal
  review.
- tests/test_catalog_review.py: 6 new tests covering re-attest mode
  (one change-entry per server, gating until all acknowledged, then
  permits), confirming normal mode still refuses an empty diff (rotation
  path is not a general bypass), and confirming reattest mode still
  enforces the blocking-risk check and the TOCTOU pin.

## show-seed-b64 hardening (Task 3)

The maintainer ran show-seed-b64 --release, saw an ambiguous prompt,
and pasted the printed PRIVATE seed into a chat believing it was public.

- cmd_show_seed_b64: passphrase prompt is now explicit ("Passphrase for
  the release signing key (the one YOU chose when generating it)"). A
  loud three-line warning banner prints to STDERR immediately before the
  seed ("!!! PRIVATE KEY BELOW..."); the seed itself stays alone on
  STDOUT so piping into pbcopy or a CI secret field still works cleanly.
- cmd_keygen: labels for both key kinds now say PUBLIC/PRIVATE explicitly
  and state safety properties inline (safe to commit vs. never commit),
  so the printed output can't be mistaken for the other key's.

## Verification

- ruff check . / ruff format --check .: clean.
- pytest: full suite green (360 passed, 1 skipped).
- Delete-the-check-and-watch-it-fail: each of can_sign()'s four gates
  (empty-diff, TOCTOU pin, blocking-risk, acknowledge-all) and the
  reattest branch itself were individually removed and confirmed to turn
  a test red, then restored -- see PR description for the exact
  failures.

Not touched: data/catalog.json (content-signed, maintainer's job via the
Console). No private key generated, requested, or committed. bcc.py
untouched (open PR #67).

tests/test_catalog_review.py

@@ -491,6 +491,98 @@ def test_sign_precondition_defaults_to_main_when_loaded_ref_unset():
     assert decision.ok is True
 
 
+# --------------------------------------------------------------------------- #
+# ReviewSession(reattest=True): key-rotation re-attestation (issue #68
+# finding 5 follow-up). After rotating bcc_core.CATALOG_PUBKEYS, the
+# existing data/catalog.json.sig no longer verifies under the new key even
+# though catalog CONTENT is unchanged -- diff_catalogs(old, new) would be
+# empty, and an empty changeset must never unlock Sign (can_sign gate 0).
+# Re-attestation mode sidesteps that correctly: instead of diffing against
+# the (now-untrustworthy) last-signed content, it treats every entry as
+# requiring a fresh acknowledgement, exactly like a brand-new catalog.
+# --------------------------------------------------------------------------- #
+def test_reattest_mode_with_unchanged_content_yields_one_change_per_entry():
+    same = _catalog(_entry(id="a"), _entry(id="b"), _entry(id="c"))
+    session = r.start_review("sha1", same, same, reattest=True)
+    assert len(session.changes) == 3
+    assert {c_.entry_id for c_ in session.changes} == {"a", "b", "c"}
+    # Presented "as if newly added" -- old_catalog plays no role here.
+    assert all(c_.status == "added" for c_ in session.changes)
+    assert all(c_.old is None for c_ in session.changes)
+
+
+def test_reattest_mode_can_sign_refuses_until_all_acknowledged_then_permits():
+    same = _catalog(_entry(id="a"), _entry(id="b"), _entry(id="c"))
+    session = r.start_review("sha1", same, same, reattest=True)
+
+    decision = r.can_sign(session, "sha1")
+    assert decision.ok is False
+    assert "acknowledged" in decision.reason.lower()
+
+    r.acknowledge_entry(session, "a")
+    r.acknowledge_entry(session, "b")
+    decision = r.can_sign(session, "sha1")
+    assert decision.ok is False  # "c" still outstanding
+    assert "acknowledged" in decision.reason.lower()
+
+    r.acknowledge_entry(session, "c")
+    decision = r.can_sign(session, "sha1")
+    assert decision.ok is True
+    assert decision.reason is None
+
+
+def test_normal_mode_with_unchanged_content_still_refuses_empty_diff():
+    """The rotation path must NOT become a general bypass of the empty-diff
+    guard: reattest=False (the default) against identical old/new catalogs
+    must behave exactly as before -- can_sign refuses with 'nothing to
+    sign', full stop."""
+    same = _catalog(_entry(id="a"), _entry(id="b"))
+    session = r.start_review("sha1", same, same)  # reattest defaults False
+    assert session.changes == []
+    decision = r.can_sign(session, "sha1")
+    assert decision.ok is False
+    assert "nothing to sign" in decision.reason.lower()
+
+
+def test_reattest_mode_still_enforces_blocking_risk_check():
+    """Re-attestation must not relax the blocking-risk gate: a
+    disallowed-command entry blocks Sign even with every entry
+    acknowledged."""
+    cat = _catalog(
+        _entry(id="a"),
+        _entry(id="evil", config={"command": "bash", "args": ["-c", "rm -rf /"]}),
+    )
+    session = r.start_review("sha1", cat, cat, reattest=True)
+    assert len(session.changes) == 2
+    r.acknowledge_entry(session, "a")
+    r.acknowledge_entry(session, "evil")
+    assert r.all_entries_acknowledged(session) is True
+    decision = r.can_sign(session, "sha1")
+    assert decision.ok is False
+    assert "blocking" in decision.reason.lower()
+
+
+def test_reattest_mode_still_enforces_toctou_pin():
+    """Re-attestation must not relax the TOCTOU blob-SHA pin: acknowledging
+    everything is not enough if the bytes moved underneath the review."""
+    same = _catalog(_entry(id="a"))
+    session = r.start_review("sha1", same, same, reattest=True)
+    r.acknowledge_entry(session, "a")
+    decision = r.can_sign(session, "sha1")
+    assert decision.ok is True  # sanity: matches when blob is unchanged
+
+    decision = r.can_sign(session, "sha2-a-new-commit-landed")
+    assert decision.ok is False
+    assert "mismatch" in decision.reason.lower() or "changed" in decision.reason.lower()
+
+
+def test_reattest_defaults_to_false():
+    """start_review()'s reattest parameter defaults to False -- normal
+    (diff-based) review is the default behaviour, never silently entered."""
+    session = r.start_review("sha1", _catalog(), _catalog(_entry()))
+    assert session.reattest is False
+
+
 # --------------------------------------------------------------------------- #
 # find_last_signed_catalog_raw: what source=main diffs against
 # --------------------------------------------------------------------------- #