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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

docs/web.md

@@ -49,6 +49,7 @@ The browser has no file system the app can reach, so files live in the page
 | Autosave to the recovery folder, File › Recover… | Autosaves are kept in the browser's local storage; a page closed or crashed with unsaved changes offers them at the next visit (File › Recover…) |
 | Leaving with unsaved changes asks first | The browser's "leave site?" prompt |
 | Recent files | This visit's files only |
+| Help › Check for Updates installs a new release (docs/updates.md) | A new deployment shows **Reload to update** in the status bar |
 
 A `.fieldcraft` model refers to its design and meshes by path. Paths in the model's own folder
 are stored relative to it, so picking or dropping the model with those files opens them again; a