feat(doctor): add town-git check for version control

Checks whether the town root (~/gt) is under git version control.
Having the town harness in git is optional but recommended for:
- Backing up personal Gas Town configuration and history
- Tracking mail and coordination beads
- Easier federation across machines

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/doctor.go

@@ -53,6 +53,7 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	d := doctor.NewDoctor()
 
 	// Register built-in checks
+	d.Register(doctor.NewTownGitCheck())
 	d.Register(doctor.NewDaemonCheck())
 	d.Register(doctor.NewBeadsDatabaseCheck())
 	d.Register(doctor.NewOrphanSessionCheck())