fix(doctor): compare HEAD against origin/main when no Releases exist

[swhitt]

Summary

Doctor reports latest release (could not reach github) even on a healthy network, because releases/latest 404s on this repo (no Releases cut). The daily update banner is silent for the same reason.

This repo ships via merges to main, so for editable git installs I switched the check over to GET /commits/main and compare against local HEAD. Pypi installs keep the existing release-tag path, so nothing changes if you start cutting Releases later.

Doctor output for an editable clone now reads:

• version: 0.1.0 (git)
• origin/main: 6d20866664ea (up to date)

Small drive-by: _repo_dir was checking (p / ".git").is_dir(), which misses linked worktrees (gitlink file). Swapped to .exists() so worktree-based reviews exercise the git path too.

Test plan

• uv run pytest (117 passing, 6 new)
• Ran the patched run_doctor() against an editable clone, shows the
  new origin/main line

---

Summary by cubic

Fixes Doctor update checks for repos without Releases by comparing local HEAD to origin/main for editable git installs. Restores the daily update banner; PyPI installs still use release tags.

• Bug Fixes
  • Doctor fetches commits/main, caches the short SHA for 24h, and shows an origin/main line with “up to date” or “update available”.
  • Falls back to release-tag checks for PyPI installs or when git probing fails.
  • _repo_dir now recognizes linked worktrees by checking .git existence, not only directories.
  • Added unit tests for SHA comparison paths, output, and worktree detection.

^{Written for commit 12c1a42. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}