feat(formula): add checksum-based auto-update for embedded formulas

Adds infrastructure to automatically update embedded formulas when
the binary is upgraded, while preserving user customizations.

Changes:
- Add CheckFormulaHealth() to detect outdated/modified/missing formulas
- Add UpdateFormulas() to safely update formulas via gt doctor --fix
- Track installed formula checksums in .beads/formulas/.installed.json
- Add FormulaCheck to gt doctor with auto-fix capability
- Compute checksums at runtime from embedded files (no build-time manifest)

Update scenarios:
- Outdated (embedded changed, user unchanged): Update automatically
- Modified (user customized): Skip with warning
- Missing (user deleted): Reinstall with message
- New (never installed): Install

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

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

internal/cmd/doctor.go

@@ -116,6 +116,7 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	d.Register(doctor.NewRepoFingerprintCheck())
 	d.Register(doctor.NewBootHealthCheck())
 	d.Register(doctor.NewBeadsDatabaseCheck())
+	d.Register(doctor.NewFormulaCheck())
 	d.Register(doctor.NewBdDaemonCheck())
 	d.Register(doctor.NewPrefixConflictCheck())
 	d.Register(doctor.NewPrefixMismatchCheck())