Fallback to gh CLI credentials if available (#1515)

Adds `gh` CLI as a GitHub credential fallback for users who already have
`gh auth login` configured locally. OpenChamber-owned OAuth credentials
remain the primary source of truth; the `gh` token is only used when no
stored OpenChamber GitHub access token exists and the fallback is not
disabled.

The fallback is implemented as a credential provider only: GitHub features
continue to use the existing Octokit/GitHub API paths for issues, pull
requests, checks, merges, and related operations. The PR does not replace
those endpoints with `gh issue` or `gh pr` CLI commands.

Server changes:
- Add `gh-cli-credential.js` to read `gh auth token` with a bounded timeout.
- Cache the `gh` token lookup for 30 seconds, including negative results,
  to avoid repeated subprocess spawning on status/polling paths.
- Hide the subprocess window on Windows via `windowsHide: true`.
- Clear the gh CLI token cache when the fallback setting changes.
- Update `getOctokitOrNull()` to prefer stored OpenChamber OAuth tokens and
  fall back to the `gh` token only when enabled.
- Add `ghCliDisabled` persistence in the existing settings file with atomic
  writes and `0o600` file permissions.
- Add `POST /api/github/auth/gh-cli` to enable or disable the fallback.
- Extend `/api/github/auth/status` with `ghCli` metadata: availability,
  disabled state, active state, and active user when applicable.

UI/runtime changes:
- Extend `GitHubAuthStatus` and `GitHubAPI` with gh CLI fallback metadata
  and toggle support.
- Add web RuntimeAPI support for toggling the gh CLI fallback through
  `runtimeFetch`, preserving active runtime/remote target behavior.
- Add deterministic VS Code unsupported handling for the gh CLI toggle.
- Update GitHub Settings to show gh CLI availability and active status.
- When gh CLI is the active auth source, show it in the connected account
  card and offer Disable instead of Disconnect.
- Keep Add Account available so users can still connect an OpenChamber OAuth
  account, which then takes priority over gh CLI.
- Add localized gh CLI settings strings across supported settings locales.

Fixes addressed during review:
- Removed unreachable UI branches in the inactive gh CLI card.
- Avoided duplicate and repeated `gh auth token` subprocess calls.
- Hardened settings file permissions for the new persisted flag.
- Routed the gh CLI toggle through the RuntimeAPI/runtimeFetch path instead
  of direct browser `fetch`.
- Added targeted tests for hidden subprocess options and negative-result
  cache behavior.
- Fixed a VS Code webview Response body typing issue that blocked type-check.

packages/ui/src/lib/i18n/messages/ko.settings.ts

@@ -1374,6 +1374,15 @@ export const settingsDict = {
   'settings.github.page.toast.disconnectFailed': 'GitHub 연결을 해제하지 못했습니다',
   'settings.github.page.toast.accountSwitched': 'GitHub 계정이 전환되었습니다',
   'settings.github.page.toast.accountSwitchFailed': 'GitHub 계정을 전환하지 못했습니다',
+  'settings.github.page.ghCli.title': 'gh CLI',
+  'settings.github.page.ghCli.activeDescription': 'gh CLI를 통해 인증됨',
+  'settings.github.page.ghCli.fallbackDescription': 'OpenChamber 인증이 제거된 경우 대체로 사용 가능',
+  'settings.github.page.ghCli.disabledDescription': '설치되어 있지만 대체로 사용되지 않음',
+  'settings.github.page.ghCli.actions.disable': '비활성화',
+  'settings.github.page.ghCli.actions.enable': '활성화',
+  'settings.github.page.toast.ghCliEnabled': 'gh CLI 대체 활성화됨',
+  'settings.github.page.toast.ghCliDisabled': 'gh CLI 대체 비활성화됨',
+  'settings.github.page.toast.ghCliUpdateFailed': 'gh CLI 설정을 업데이트하지 못했습니다',
   'settings.notifications.page.delivery.title': '알림',
   'settings.notifications.page.delivery.enableAria': '알림 활성화',
   'settings.notifications.page.delivery.enableLabel': '알림 활성화',