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

CONTRIBUTING.pt-BR.md

@@ -23,13 +23,11 @@ No Linux os testes geram imagens com `mtools`, `ntfs-3g`, `e2fsprogs`, `exfatpro
 
 Para as imagens de referência externas (BitLocker, FileVault 2, dfvfs, AFF4), rode `scripts/fetch-testdata.sh`; sem elas esses testes são pulados. O teste de BitLocker leva cerca de 75 s.
 
-O app desktop assina os pacotes de atualização: o `tauri.conf.json` tem `createUpdaterArtifacts` ligado, então um `tauri build` local precisa de `TAURI_SIGNING_PRIVATE_KEY` (qualquer chave feita com `tauri signer generate`; a chave da release fica só nos segredos do repositório). Para um pacote local rápido, passe `--config '{"bundle":{"createUpdaterArtifacts":false}}'`.
-
 ## Releases e Versionamento
 
 As versões seguem estritamente o **Versionamento Semântico 2.0.0** (`MAJOR.MINOR.PATCH`), sincronizadas entre `Cargo.toml`, `apps/gui/src-tauri/Cargo.toml` e `apps/gui/src-tauri/tauri.conf.json` (validado por `scripts/check.sh`).
 
-Enviar uma tag `v*` faz o CI do Gitea criar a release (as notas vêm do `CHANGELOG.md`, então atualize-o antes), anexar o CLI para Linux (x86_64 e aarch64), o app desktop para Linux (x86_64 e aarch64) e Windows, e montar o `latest.json` do atualizador.
+Enviar uma tag `v*` faz o CI do Gitea criar a release (as notas vêm do `CHANGELOG.md`, então atualize-o antes) e anexar o CLI para Linux (x86_64 e aarch64) e o app desktop para Linux (x86_64 e aarch64) e Windows.
 
 Não há build para macOS: ele exige o SDK e a assinatura da Apple, que um runner Linux não tem.