[Fix] Resolve implicit repository branches from origin HEAD

[daniel-lxs]

What changed

• Prefer the fetched local origin/HEAD when a task does not explicitly select a branch.
• Fall back to the remote HEAD lookup with a bounded retry.
• Verify remote-tracking refs before checkout and fail with a clear diagnostic when neither HEAD nor stored metadata points to an existing branch.
• Add regression coverage for stale metadata, missing local HEAD, verified stored defaults, and unresolved defaults.

Why

All-repositories tasks intentionally omit a branch. If stored repository metadata was stale and the remote HEAD lookup returned no result, the worker previously fell through to that stale value and attempted a checkout such as origin/main even when the repository default was develop.

This keeps explicit branch selections unchanged while making implicit default-branch resolution resilient to stale metadata and transient provider lookups.

Addresses the branch-checkout failure reported in #630.

Validation

• pnpm exec dotenvx run -f .env.test -- pnpm --filter @roomote/worker exec vitest run src/workspace/__tests__ src/commands/setup/__tests__/workspace.test.ts (69 tests)
• pnpm --filter @roomote/worker check-types
• pnpm --filter @roomote/worker lint
• Full pre-push suite: oxlint, residual lint, fast typecheck, and knip
• Real clone of easonLiangWorldedtech/Roomote with deliberately stale main metadata checked out develop
• Rebuilt local worker release archive and confirmed the packaged worker contains the new resolver

[roomote-roomote]

1 issue outstanding. See task

• apps/worker/src/workspace/workspace-manager.ts:854 git fetch --all does not refresh the symbolic refs/remotes/origin/HEAD. After a repository changes its default branch while the old branch still exists, this stale local ref is verified and selected before ls-remote, so an implicit checkout remains on the old default branch. Refresh the remote HEAD or prefer the remote lookup before trusting the cached ref.

_{Reviewed 8423323}