fix: canonicalize path case on macOS for git worktree operations (GH#880)

bd sync fails with exit status 128 when the daemon is started from a
terminal with different path casing than what git has stored. This
happens on macOS case-insensitive filesystem when directory names
are renamed (e.g., MyProject to myproject) but terminal sessions
retain the old casing.

The fix uses realpath(1) on macOS to get the true filesystem case
when canonicalizing paths:
- CanonicalizePath() now calls realpath on macOS
- git.GetRepoRoot() canonicalizes repoRoot via canonicalizeCase()
- syncbranch.GetRepoRoot() uses utils.CanonicalizePath()

This ensures git worktree paths match exactly, preventing the
exit status 128 errors from git operations.

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

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

internal/git/gitdir.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"sync"
 )
@@ -62,11 +63,17 @@ func initGitContext() {
 	// Derive isWorktree from comparing absolute paths
 	gitCtx.isWorktree = absGitDir != absCommon
 
-	// Process repoRoot: normalize Windows paths and resolve symlinks
+	// Process repoRoot: normalize Windows paths, resolve symlinks,
+	// and canonicalize case on case-insensitive filesystems (GH#880).
+	// This is critical for git worktree operations which string-compare paths.
 	repoRoot := NormalizePath(repoRootRaw)
 	if resolved, err := filepath.EvalSymlinks(repoRoot); err == nil {
 		repoRoot = resolved
 	}
+	// Canonicalize case on macOS/Windows (GH#880)
+	if canonicalized := canonicalizeCase(repoRoot); canonicalized != "" {
+		repoRoot = canonicalized
+	}
 	gitCtx.repoRoot = repoRoot
 }
 
@@ -173,6 +180,27 @@ func GetRepoRoot() string {
 	return ctx.repoRoot
 }
 
+// canonicalizeCase resolves a path to its true filesystem case on
+// case-insensitive filesystems (macOS/Windows). This is needed because
+// git operations string-compare paths exactly - a path with wrong case
+// will fail even though it points to the same location. (GH#880)
+//
+// On macOS, uses realpath(1) which returns the canonical case.
+// Returns empty string if resolution fails or isn't needed.
+func canonicalizeCase(path string) string {
+	if runtime.GOOS == "darwin" {
+		// Use realpath to get canonical path with correct case
+		cmd := exec.Command("realpath", path)
+		output, err := cmd.Output()
+		if err == nil {
+			return strings.TrimSpace(string(output))
+		}
+	}
+	// Windows: filepath.EvalSymlinks already handles case
+	// Linux: case-sensitive, no canonicalization needed
+	return ""
+}
+
 // NormalizePath converts Git's Windows path formats to native format.
 // Git on Windows may return paths like /c/Users/... or C:/Users/...
 // This function converts them to native Windows format (C:\Users\...).