feat(doctor): add health check framework (gt-f9x.4)

Add doctor package with:
- Check interface for implementing health checks
- CheckContext for passing context to checks
- CheckResult and CheckStatus types
- Report with summary and pretty printing
- Doctor runner with Run() and Fix() methods
- BaseCheck and FixableCheck for easy check implementation
- CLI command: gt doctor [--fix] [--verbose] [--rig <name>]

Built-in checks will be added in:
- gt-f9x.5: Town-level checks (config, state, mail, rigs)
- gt-f9x.6: Rig-level checks (refinery, clones, gitignore)

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

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

internal/doctor/errors.go

@@ -0,0 +1,9 @@
+package doctor
+
+import "errors"
+
+// Common errors
+var (
+	// ErrCannotFix is returned when a check does not support auto-fix.
+	ErrCannotFix = errors.New("check does not support auto-fix")
+)