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.

README.md

@@ -19,6 +19,65 @@ Pre-built self-contained binaries are attached to every [GitHub Release](../../r
 
 > **macOS Gatekeeper note:** the app is not notarized. On first launch, right-click → **Open**, or run `xattr -cr /Applications/BetterClaudeConfig.app` in a terminal.
 
+## Verifying your download
+
+BCC isn't code-signed — there's no budget for a paid certificate (macOS
+Developer ID, Windows Authenticode). Instead, every release publishes a
+`SHA256SUMS` file listing the checksum of each archive, detached-signed with
+Ed25519 as `SHA256SUMS.sig`. Both are attached to the release alongside the
+binaries.
+
+**What this proves:** the file you downloaded is byte-for-byte what we
+published, and the manifest itself was signed by our release key.
+
+**What this does NOT do:** it does not make the binary "safe," and it does
+**not** remove the macOS Gatekeeper or Windows SmartScreen warning — those
+are only suppressed by a paid OS-vendor certificate, which this project
+doesn't have. Verifying checksums is about detecting tampering in transit or
+on a mirror, not about vouching for the software.
+
+**Release signing public key** (Ed25519, base64, raw 32 bytes):
+
+```
+<PLACEHOLDER — AJ: paste the public key from the Catalog Console (#62) here>
+```
+
+### macOS / Linux
+
+```bash
+# From inside the folder you downloaded the release files into:
+sha256sum -c SHA256SUMS
+```
+
+If your `sha256sum` complains about missing files, download `SHA256SUMS`
+into the same directory as the archive you downloaded — it lists every
+platform's archive, and only the one(s) present will be checked.
+
+To also verify the manifest's signature (optional, requires Python +
+`pip install cryptography` and a checkout of this repo):
+
+```bash
+python3 scripts/sign_checksums.py verify \
+  --sums SHA256SUMS --sig SHA256SUMS.sig \
+  --pubkey-b64 "<the public key above>"
+```
+
+### Windows (PowerShell)
+
+```powershell
+Get-FileHash .\BetterClaudeConfig-Windows.zip -Algorithm SHA256
+```
+
+Compare the printed hash (case-insensitively) against the matching line in
+`SHA256SUMS`.
+
+### If a release has no `SHA256SUMS.sig`
+
+The signing key is a repo secret that has to be configured manually; if a
+release is missing the `.sig` file, the checksums themselves are still
+valid and safe to check against — the release workflow only skips signing,
+never checksum generation.
+
 ## Run from source
 
 ```bash
@@ -92,6 +151,7 @@ file is also listed, marked *legacy*, so you can copy them over.
 - `test_core.py` — unit suite for the core (`python test_core.py`).
 - `bcc.spec` — PyInstaller build spec (cross-platform).
 - `scripts/build_icons.py` — regenerates `icons/app.icns` and `icons/app.ico` from source PNGs.
+- `scripts/sign_checksums.py` — generates and Ed25519-signs the release `SHA256SUMS` manifest (see [Verifying your download](#verifying-your-download)).
 
 ## Building from source