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/web/server/lib/github/auth.js

@@ -305,3 +305,41 @@ export function getGitHubScopes() {
 }
 
 export const GITHUB_AUTH_FILE = STORAGE_FILE;
+
+export function isGhCliDisabled() {
+  try {
+    if (fs.existsSync(SETTINGS_FILE)) {
+      const parsed = JSON.parse(fs.readFileSync(SETTINGS_FILE, 'utf8'));
+      return Boolean(parsed?.ghCliDisabled);
+    }
+  } catch {
+    // ignore
+  }
+  return false;
+}
+
+export function setGhCliDisabled(disabled) {
+  ensureStorageDir();
+  let settings = {};
+  try {
+    if (fs.existsSync(SETTINGS_FILE)) {
+      settings = JSON.parse(fs.readFileSync(SETTINGS_FILE, 'utf8')) || {};
+    }
+  } catch {
+    // ignore
+  }
+  settings.ghCliDisabled = Boolean(disabled);
+  const tmpFile = `${SETTINGS_FILE}.${process.pid}.${Date.now()}.tmp`;
+  fs.writeFileSync(tmpFile, JSON.stringify(settings, null, 2), 'utf8');
+  try {
+    fs.chmodSync(tmpFile, 0o600);
+  } catch {
+    // best-effort
+  }
+  fs.renameSync(tmpFile, SETTINGS_FILE);
+  try {
+    fs.chmodSync(SETTINGS_FILE, 0o600);
+  } catch {
+    // best-effort
+  }
+}