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>

internal/git/gitdir.go

@@ -5,6 +5,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strings"
+	"sync"
 )
 
 // GetGitDir returns the actual .git directory path for the current repository.
@@ -52,25 +53,43 @@ func GetGitHeadPath() (string, error) {
 	return filepath.Join(gitDir, "HEAD"), nil
 }
 
+// isWorktreeOnce ensures we only check worktree status once per process.
+// This is safe because worktree status cannot change during a single command execution.
+var (
+	isWorktreeOnce   sync.Once
+	isWorktreeResult bool
+)
+
 // IsWorktree returns true if the current directory is in a Git worktree.
 // This is determined by comparing --git-dir and --git-common-dir.
+// The result is cached after the first call since worktree status doesn't
+// change during a single command execution.
 func IsWorktree() bool {
+	isWorktreeOnce.Do(func() {
+		isWorktreeResult = isWorktreeUncached()
+	})
+	return isWorktreeResult
+}
+
+// isWorktreeUncached performs the actual worktree check without caching.
+// Called once by IsWorktree and cached for subsequent calls.
+func isWorktreeUncached() bool {
 	gitDir := getGitDirNoError("--git-dir")
 	if gitDir == "" {
 		return false
 	}
-	
+
 	commonDir := getGitDirNoError("--git-common-dir")
 	if commonDir == "" {
 		return false
 	}
-	
+
 	absGit, err1 := filepath.Abs(gitDir)
 	absCommon, err2 := filepath.Abs(commonDir)
 	if err1 != nil || err2 != nil {
 		return false
 	}
-	
+
 	return absGit != absCommon
 }
 
@@ -103,13 +122,28 @@ func GetMainRepoRoot() (string, error) {
 	return mainRepoRoot, nil
 }
 
+// repoRootOnce ensures we only get repo root once per process.
+var (
+	repoRootOnce   sync.Once
+	repoRootResult string
+)
+
 // GetRepoRoot returns the root directory of the current git repository.
 // Returns empty string if not in a git repository.
 //
 // This function is worktree-aware and handles Windows path normalization
 // (Git on Windows may return paths like /c/Users/... or C:/Users/...).
 // It also resolves symlinks to get the canonical path.
+// The result is cached after the first call.
 func GetRepoRoot() string {
+	repoRootOnce.Do(func() {
+		repoRootResult = getRepoRootUncached()
+	})
+	return repoRootResult
+}
+
+// getRepoRootUncached performs the actual repo root lookup without caching.
+func getRepoRootUncached() string {
 	cmd := exec.Command("git", "rev-parse", "--show-toplevel")
 	output, err := cmd.Output()
 	if err != nil {