bakery: rotate signing key, fix broken index-signature verification

The old bakery-signing-key.minisign-sec on hestia was password-encrypted
and the password was lost, so scripts/gen-index.sh never actually signed
index.json (silent no-op warning). bakery/src/manifest.rs (0.3.0+) hard-
requires that signature, so every bakery command has been failing with
'fetching index.json.minisig — the index must be signed before it can be
trusted' since the signing enforcement shipped.

Generated a new no-password minisign keypair on hestia
(~/.secrets/bakery-signing-key-2.minisign-sec), updated the hardcoded
PUBKEY in manifest.rs and get.sh to match, wired
BAKERY_MINISIGN_SEC_KEY_PATH as a Forgejo Actions secret so future CI
releases sign automatically, and manually signed+published the current
index.json on hestia so bakery works immediately.

bakery/src/manifest.rs

@@ -17,7 +17,7 @@ const CACHE_MAX_AGE: Duration = Duration::from_secs(24 * 3600);
 /// bytes are trusted or parsed. This is the single control point: the
 /// per-artifact `sha256` fields and `post_install` hook strings all live
 /// inside `index.json` itself, so a valid signature transitively covers them.
-const PUBKEY: &str = "RWRh2Zr5SUinvVFCtD7S7HwGjfrye6j31Xq2mYXRdkGFDWe3yHF7W11K";
+const PUBKEY: &str = "RWTBR8w/IJ+jaylOv80b52DzekKbSR2CvOVGvzB0ipGBaMhJPAOiEWq8";
 
 /// Verify `bytes` against `sig_text` (the contents of an `index.json.minisig`
 /// file) using the pinned [`PUBKEY`]. Returns an error on any failure —