Merge PR #665: fix(setup): preserve symlinks in atomicWriteFile

Add ResolveForWrite helper that resolves symlinks before writing, so
atomic writes go to the symlink target instead of replacing the symlink.

This prevents `bd setup claude` from overwriting nix/home-manager
managed ~/.claude/settings.json symlinks.

Closes #665

Co-authored-by: qmx <qmx@qmx.me>

cmd/bd/setup/utils.go

@@ -4,12 +4,20 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+
+	"github.com/steveyegge/beads/internal/utils"
 )
 
 // atomicWriteFile writes data to a file atomically using a unique temporary file.
 // This prevents race conditions when multiple processes write to the same file.
+// If path is a symlink, writes to the resolved target (preserving the symlink).
 func atomicWriteFile(path string, data []byte) error {
-	dir := filepath.Dir(path)
+	targetPath, err := utils.ResolveForWrite(path)
+	if err != nil {
+		return fmt.Errorf("resolve path: %w", err)
+	}
+
+	dir := filepath.Dir(targetPath)
 
 	// Create unique temp file in same directory
 	tmpFile, err := os.CreateTemp(dir, ".*.tmp")
@@ -38,7 +46,7 @@ func atomicWriteFile(path string, data []byte) error {
 	}
 
 	// Atomic rename
-	if err := os.Rename(tmpPath, path); err != nil {
+	if err := os.Rename(tmpPath, targetPath); err != nil {
 		_ = os.Remove(tmpPath) // Best effort cleanup
 		return fmt.Errorf("rename temp file: %w", err)
 	}

cmd/bd/setup/utils_test.go

@@ -67,6 +67,45 @@ func TestAtomicWriteFile(t *testing.T) {
 	}
 }
 
+func TestAtomicWriteFile_PreservesSymlink(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	// Create target file
+	target := filepath.Join(tmpDir, "target.txt")
+	if err := os.WriteFile(target, []byte("original"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create symlink
+	link := filepath.Join(tmpDir, "link.txt")
+	if err := os.Symlink(target, link); err != nil {
+		t.Fatal(err)
+	}
+
+	// Write via symlink
+	if err := atomicWriteFile(link, []byte("updated")); err != nil {
+		t.Fatalf("atomicWriteFile failed: %v", err)
+	}
+
+	// Verify symlink still exists
+	info, err := os.Lstat(link)
+	if err != nil {
+		t.Fatalf("failed to lstat link: %v", err)
+	}
+	if info.Mode()&os.ModeSymlink == 0 {
+		t.Error("symlink was replaced with regular file")
+	}
+
+	// Verify target was updated
+	data, err := os.ReadFile(target)
+	if err != nil {
+		t.Fatalf("failed to read target: %v", err)
+	}
+	if string(data) != "updated" {
+		t.Errorf("target content = %q, want %q", string(data), "updated")
+	}
+}
+
 func TestDirExists(t *testing.T) {
 	tmpDir := t.TempDir()