Add a self-diagnostic command that validates the environment:

parsec doctor

Checks:

• git version and worktree support
• Token configuration (per-host, env vars)
• Tracker connectivity (Jira/GitHub/GitLab API reachable)
• Shell integration installed (parsec init)
• Tab completions configured
• Remote access (git push test)

Prints ✓/✗ for each check with actionable fix instructions.
Crucial for smooth first-time setup experience.