feat(update): unify the updater with Sabiá's Gitea-based mechanism

Replaces the signed tauri-plugin-updater (latest.json manifest, minisign
key pair) with the same simple mechanism Sabiá already uses: query the
Gitea Releases API directly, download the matching platform archive,
verify its SHA-256 checksum, and swap the binary in place. The CI job
that built the old latest.json manifest is removed in the next commit,
alongside an unrelated runner-routing change to the same workflow file.

- New guara-core::update module (check/apply), ported from
  sabia-core::update and adapted to Guará's own release asset naming
  (guara-<tag>-<platform>.tar.gz, matching scripts/package-cli.sh).
- New Engine::update_check / Engine::update_apply, wired into the RPC
  service for the browser UI.
- New CLI subcommand: "guara update" (check) and "guara update --apply"
  (download, verify, replace), mirroring "sabia update".
- GUI: replaced the Tauri plugin-updater flow (download_and_install,
  update-progress events, in-app restart) with a call to the new
  update_check/update_apply commands. The desktop app can no longer
  self-install in one click — it now points the user to the Releases
  page, same as Sabiá's desktop app already does.
- Removed tauri-plugin-updater, createUpdaterArtifacts and the
  updater.pubkey config, the signing-key loading in build-gui.sh, the
  CI step testing the old latest.json generator, and the now-unused
  make-latest-json.py/test_make_latest_json.py scripts.
- Documented the break for existing installs in CHANGELOG.md /
  CHANGELOG.pt-BR.md: versions up to 1.3.2 won't find this release
  automatically and need one manual download.

Trade-off accepted knowingly: SHA-256 verifies integrity against
Gitea's own API but, unlike the retired signed mechanism, does not
protect against a compromised Gitea server independently of it. This
is Lucas's own self-hosted instance, not a third-party mirror.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VAyr8d7HuAdUGB6aA5xmDD

apps/gui/src-tauri/tauri.conf.json

@@ -34,18 +34,6 @@
       "icons/128x128@2x.png",
       "icons/icon.icns",
       "icons/icon.ico"
-    ],
-    "createUpdaterArtifacts": true
-  },
-  "plugins": {
-    "updater": {
-      "pubkey": "dW50cnVzdGVkIGNvbW1lbnQ6IG1pbmlzaWduIHB1YmxpYyBrZXk6IDhENzRCRUU5NEI3QTAxQzgKUldUSUFYcEw2YjUwamFzMnpXM3ZKTEZPN1JIeXdLL3Q5SEF1M1FxQ284WFM0UWNsdlAyNUloWTEK",
-      "endpoints": [
-        "https://git.lucasrguerra.dev.br/lucasrguerra/guara/releases/latest/download/latest.json"
-      ],
-      "windows": {
-        "installMode": "passive"
-      }
-    }
+    ]
   }
 }