Sign release checksums with Ed25519 (#63)

Publish SHA256SUMS for every release archive and sign it with a
detached Ed25519 signature (SHA256SUMS.sig), since paid code signing
(macOS Developer ID, Windows Authenticode) and Sigstore keyless (needs
a Fulcio-trusted OIDC issuer; self-hosted Gitea isn't one) are both
out of budget/scope.

- scripts/sign_checksums.py: dependency-light (cryptography only)
  helper to hash a directory of files into a sha256sum(1)-compatible
  SHA256SUMS manifest, sign it (domain-separated: b"bcc-release-v1|"
  + raw manifest bytes), and verify a signature. CLI has generate/
  sign/verify subcommands; verify doubles as the check path.
- tests/test_checksums.py: 15 unit + CLI-subprocess tests covering
  hashing, manifest formatting, sign/verify roundtrip, tamper
  detection, wrong-key rejection, domain-separation, and the
  no-key-provided failure path (must error, never write an empty/
  bogus .sig).
- .github/workflows/release.yml: Publish Release job now checks out
  the repo, flattens build artifacts, generates SHA256SUMS, and signs
  it from the RELEASE_SIGNING_KEY secret (base64 raw Ed25519 seed) if
  present. If the secret is absent, the release still publishes with
  a loud ::warning:: and no .sig — it never fails the release or
  publishes a bogus signature.
- README.md: new 'Verifying your download' section with the (still
  placeholder) public key, sha256sum -c / Get-FileHash commands, and
  an explicit statement that this does not remove Gatekeeper/
  SmartScreen warnings.
- requirements-dev.txt / ci.yml: add cryptography as a dev/test
  dependency for the new script and its tests.

Touches no files from bcc_core.py / tests/test_core.py /
pyproject.toml / bcc.spec to avoid colliding with concurrent work on
those files.

.github/workflows/release.yml

@@ -107,11 +107,72 @@ jobs:
       contents: write
 
     steps:
+      # Needed for scripts/sign_checksums.py — the release job otherwise
+      # only downloads build artifacts, it doesn't check out the repo.
+      - name: Checkout
+        uses: actions/checkout@v4
+
       - name: Download all artifacts
         uses: actions/download-artifact@v3
         with:
           path: artifacts
 
+      - name: Set up Python 3.12
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      # download-artifact@v3 nests each artifact under a directory named
+      # after it (artifacts/<name>/<name>). Flatten into one directory so
+      # SHA256SUMS lists plain filenames, matching what `sha256sum -c`
+      # expects when run from inside an extracted release download.
+      - name: Collect release files
+        run: |
+          mkdir -p release-files
+          find artifacts -type f -exec cp {} release-files/ \;
+          ls -la release-files
+
+      - name: Generate SHA256SUMS
+        run: python3 scripts/sign_checksums.py generate release-files --out release-files/SHA256SUMS
+
+      # ── Sign the checksum manifest (best-effort) ──────────────────────
+      #
+      # BCC binaries are not code-signed (no budget for a paid cert). This
+      # is the free half: a checksum manifest, detached-signed with
+      # Ed25519, so a tampered download is detectable by anyone who
+      # checks. It does NOT remove Gatekeeper/SmartScreen warnings.
+      #
+      # The private key is a repo secret (RELEASE_SIGNING_KEY, base64 raw
+      # Ed25519 seed) generated via the Catalog Console (#62). If it's not
+      # set, we still publish the release — just without a .sig — rather
+      # than fail the release outright.
+      - name: Check for signing key
+        id: signing
+        run: |
+          if [ -n "${{ secrets.RELEASE_SIGNING_KEY }}" ]; then
+            echo "has_key=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "has_key=false" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Install signing dependencies
+        if: steps.signing.outputs.has_key == 'true'
+        run: pip install cryptography
+
+      - name: Sign SHA256SUMS
+        if: steps.signing.outputs.has_key == 'true'
+        env:
+          RELEASE_SIGNING_KEY: ${{ secrets.RELEASE_SIGNING_KEY }}
+        run: |
+          python3 scripts/sign_checksums.py sign \
+            --sums release-files/SHA256SUMS \
+            --out release-files/SHA256SUMS.sig
+
+      - name: Warn — release will be unsigned
+        if: steps.signing.outputs.has_key != 'true'
+        run: |
+          echo "::warning::RELEASE_SIGNING_KEY secret is not set — this release is being published WITHOUT a signed SHA256SUMS.sig. Add the secret (base64 raw Ed25519 seed, generated via the Catalog Console, #62) before the next tag."
+
       - name: Create GitHub Release
         uses: softprops/action-gh-release@v2
         with:
@@ -119,7 +180,9 @@ jobs:
           draft: false
           prerelease: false
           generate_release_notes: false
-          files: artifacts/**/*
+          files: |
+            artifacts/**/*
+            release-files/SHA256SUMS*
           body: |
             ## Better Claude Config ${{ github.ref_name }}
 
@@ -139,5 +202,8 @@ jobs:
             xattr -cr /Applications/BetterClaudeConfig.app
             ```
 
+            ### Verifying your download
+            Every release includes `SHA256SUMS` (and, when the signing key is configured, a detached `SHA256SUMS.sig`). See [Verifying your download](https://git.avezzano.io/the_og/better-claude-config#verifying-your-download) in the README for commands. This proves you got the file we published — it does not remove Gatekeeper/SmartScreen warnings.
+
             ### Requirements
             No Python installation needed — the app is self-contained.