Add TOML support for formulas (gt-xmyha)

- Add TOML parsing using BurntSushi/toml
- Update formula loader to try .toml first, fall back to .json
- Add `bd formula convert` command for JSON→TOML migration
- Multi-line string support for readable descriptions
- Cache git worktree/reporoot checks for performance

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

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

go.mod

@@ -22,6 +22,7 @@ require (
 )
 
 require (
+	github.com/BurntSushi/toml v1.6.0 // indirect
 	github.com/atotto/clipboard v0.1.4 // indirect
 	github.com/aymanbagabas/go-osc52/v2 v2.0.1 // indirect
 	github.com/catppuccin/go v0.3.0 // indirect