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/gen/schemas/desktop-schema.json

@@ -2281,60 +2281,6 @@
           "type": "string",
           "const": "dialog:deny-save",
           "markdownDescription": "Denies the save command without any pre-configured scope."
-        },
-        {
-          "description": "This permission set configures which kind of\nupdater functions are exposed to the frontend.\n\n#### Granted Permissions\n\nThe full workflow from checking for updates to installing them\nis enabled.\n\n\n#### This default permission set includes:\n\n- `allow-check`\n- `allow-download`\n- `allow-install`\n- `allow-download-and-install`",
-          "type": "string",
-          "const": "updater:default",
-          "markdownDescription": "This permission set configures which kind of\nupdater functions are exposed to the frontend.\n\n#### Granted Permissions\n\nThe full workflow from checking for updates to installing them\nis enabled.\n\n\n#### This default permission set includes:\n\n- `allow-check`\n- `allow-download`\n- `allow-install`\n- `allow-download-and-install`"
-        },
-        {
-          "description": "Enables the check command without any pre-configured scope.",
-          "type": "string",
-          "const": "updater:allow-check",
-          "markdownDescription": "Enables the check command without any pre-configured scope."
-        },
-        {
-          "description": "Enables the download command without any pre-configured scope.",
-          "type": "string",
-          "const": "updater:allow-download",
-          "markdownDescription": "Enables the download command without any pre-configured scope."
-        },
-        {
-          "description": "Enables the download_and_install command without any pre-configured scope.",
-          "type": "string",
-          "const": "updater:allow-download-and-install",
-          "markdownDescription": "Enables the download_and_install command without any pre-configured scope."
-        },
-        {
-          "description": "Enables the install command without any pre-configured scope.",
-          "type": "string",
-          "const": "updater:allow-install",
-          "markdownDescription": "Enables the install command without any pre-configured scope."
-        },
-        {
-          "description": "Denies the check command without any pre-configured scope.",
-          "type": "string",
-          "const": "updater:deny-check",
-          "markdownDescription": "Denies the check command without any pre-configured scope."
-        },
-        {
-          "description": "Denies the download command without any pre-configured scope.",
-          "type": "string",
-          "const": "updater:deny-download",
-          "markdownDescription": "Denies the download command without any pre-configured scope."
-        },
-        {
-          "description": "Denies the download_and_install command without any pre-configured scope.",
-          "type": "string",
-          "const": "updater:deny-download-and-install",
-          "markdownDescription": "Denies the download_and_install command without any pre-configured scope."
-        },
-        {
-          "description": "Denies the install command without any pre-configured scope.",
-          "type": "string",
-          "const": "updater:deny-install",
-          "markdownDescription": "Denies the install command without any pre-configured scope."
         }
       ]
     },