Fix bd-73: Add git worktree detection and warnings

- Implement robust worktree detection using git-dir vs git-common-dir comparison
- Add prominent warning when daemon mode is active in a worktree
- Warn in 3 places: initial connection, auto-start, and daemon start command
- Show shared database path and clarify BEADS_AUTO_START_DAEMON behavior
- Document limitations and solutions in README.md and AGENTS.md
- Add comprehensive tests for detection and path truncation

Fixes #55

Amp-Thread-ID: https://ampcode.com/threads/T-254eb9e3-1a42-42d7-afdf-b7ca2d2dcb8b
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/worktree.go

@@ -0,0 +1,93 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+// isGitWorktree detects if the current directory is in a git worktree
+// by comparing --git-dir and --git-common-dir (canonical detection method)
+func isGitWorktree() bool {
+	gitDir := gitRevParse("--git-dir")
+	if gitDir == "" {
+		return false
+	}
+	
+	commonDir := gitRevParse("--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
+}
+
+// gitRevParse runs git rev-parse with the given flag and returns the trimmed output
+func gitRevParse(flag string) string {
+	out, err := exec.Command("git", "rev-parse", flag).Output()
+	if err != nil {
+		return ""
+	}
+	return strings.TrimSpace(string(out))
+}
+
+// getWorktreeGitDir returns the .git directory path for a worktree
+// Returns empty string if not in a git repo or not a worktree
+func getWorktreeGitDir() string {
+	cmd := exec.Command("git", "rev-parse", "--git-dir")
+	out, err := cmd.Output()
+	if err != nil {
+		return ""
+	}
+	return strings.TrimSpace(string(out))
+}
+
+// warnWorktreeDaemon prints a warning if using daemon with worktrees
+// Call this only when daemon mode is actually active (connected)
+func warnWorktreeDaemon(dbPathForWarning string) {
+	if !isGitWorktree() {
+		return
+	}
+	
+	gitDir := getWorktreeGitDir()
+	beadsDir := filepath.Dir(dbPathForWarning)
+	if beadsDir == "." || beadsDir == "" {
+		beadsDir = dbPathForWarning
+	}
+	
+	fmt.Fprintln(os.Stderr)
+	fmt.Fprintln(os.Stderr, "╔══════════════════════════════════════════════════════════════════════════╗")
+	fmt.Fprintln(os.Stderr, "║ WARNING: Git worktree detected with daemon mode                         ║")
+	fmt.Fprintln(os.Stderr, "╠══════════════════════════════════════════════════════════════════════════╣")
+	fmt.Fprintln(os.Stderr, "║ Git worktrees share the same .beads directory, which can cause the      ║")
+	fmt.Fprintln(os.Stderr, "║ daemon to commit/push to the wrong branch.                               ║")
+	fmt.Fprintln(os.Stderr, "║                                                                          ║")
+	fmt.Fprintf(os.Stderr, "║ Shared database: %-55s ║\n", truncateForBox(beadsDir, 55))
+	fmt.Fprintf(os.Stderr, "║ Worktree git dir: %-54s ║\n", truncateForBox(gitDir, 54))
+	fmt.Fprintln(os.Stderr, "║                                                                          ║")
+	fmt.Fprintln(os.Stderr, "║ RECOMMENDED SOLUTIONS:                                                   ║")
+	fmt.Fprintln(os.Stderr, "║   1. Use --no-daemon flag:    bd --no-daemon <command>                   ║")
+	fmt.Fprintln(os.Stderr, "║   2. Disable daemon mode:     export BEADS_NO_DAEMON=1                   ║")
+	fmt.Fprintln(os.Stderr, "║                                                                          ║")
+	fmt.Fprintln(os.Stderr, "║ Note: BEADS_AUTO_START_DAEMON=false only prevents auto-start;           ║")
+	fmt.Fprintln(os.Stderr, "║       you can still connect to a running daemon.                         ║")
+	fmt.Fprintln(os.Stderr, "╚══════════════════════════════════════════════════════════════════════════╝")
+	fmt.Fprintln(os.Stderr)
+}
+
+// truncateForBox truncates a path to fit in the warning box
+func truncateForBox(path string, maxLen int) string {
+	if len(path) <= maxLen {
+		return path
+	}
+	// Truncate with ellipsis
+	return "..." + path[len(path)-(maxLen-3):]
+}