Release: sign SHA256SUMS.txt with the release key; docs/updates.md (#231)

The release job signs SHA256SUMS.txt into SHA256SUMS.txt.minisig with rsign2 when the
FIELDCRAFT_SIGNING_KEY secret exists, and checks the signature against the public key the app
is built with; without the secret the draft is unsigned, the run warns and the notes say the
app cannot install that release itself. docs/releasing.md says how the key is set up, kept
and rotated; docs/updates.md describes checking, verifying and installing, the web app's
Reload to update, and FIELDCRAFT_UPDATE_HOST for tests. privacy.md, control-protocol.md,
web.md and the roadmap say what changed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

.github/release-notes.md

@@ -21,6 +21,8 @@ modal problems with its own solver. AI agents can drive it over MCP (`fieldcraft
 To start with a sample model, run the app with `--sample cantilever` (macOS:
 `/Applications/FieldCraft.app/Contents/MacOS/FieldCraft --sample cantilever`).
 
+**Updates.** {update_signing}
+
 **Signing.** {macos_signing} The Linux and Windows builds are not signed; on Windows, SmartScreen
 may say "Windows protected your PC": choose **More info**, then **Run anyway**.