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 itself.

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()
 

internal/utils/path.go

@@ -69,6 +69,23 @@ func FindMoleculesJSONLInDir(dbDir string) string {
 	return ""
 }
 
+// ResolveForWrite returns the path to write to, resolving symlinks.
+// If path is a symlink, returns the resolved target path.
+// If path doesn't exist, returns path unchanged (new file).
+func ResolveForWrite(path string) (string, error) {
+	info, err := os.Lstat(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return path, nil
+		}
+		return "", err
+	}
+	if info.Mode()&os.ModeSymlink != 0 {
+		return filepath.EvalSymlinks(path)
+	}
+	return path, nil
+}
+
 // CanonicalizePath converts a path to its canonical form by:
 // 1. Converting to absolute path
 // 2. Resolving symlinks

internal/utils/path_test.go

@@ -179,3 +179,54 @@ func TestCanonicalizePathSymlink(t *testing.T) {
 		}
 	}
 }
+
+func TestResolveForWrite(t *testing.T) {
+	t.Run("regular file", func(t *testing.T) {
+		tmpDir := t.TempDir()
+		file := filepath.Join(tmpDir, "regular.txt")
+		if err := os.WriteFile(file, []byte("test"), 0644); err != nil {
+			t.Fatal(err)
+		}
+
+		got, err := ResolveForWrite(file)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if got != file {
+			t.Errorf("got %q, want %q", got, file)
+		}
+	})
+
+	t.Run("symlink", func(t *testing.T) {
+		tmpDir := t.TempDir()
+		target := filepath.Join(tmpDir, "target.txt")
+		if err := os.WriteFile(target, []byte("test"), 0644); err != nil {
+			t.Fatal(err)
+		}
+		link := filepath.Join(tmpDir, "link.txt")
+		if err := os.Symlink(target, link); err != nil {
+			t.Fatal(err)
+		}
+
+		got, err := ResolveForWrite(link)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if got != target {
+			t.Errorf("got %q, want %q", got, target)
+		}
+	})
+
+	t.Run("non-existent", func(t *testing.T) {
+		tmpDir := t.TempDir()
+		newFile := filepath.Join(tmpDir, "new.txt")
+
+		got, err := ResolveForWrite(newFile)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if got != newFile {
+			t.Errorf("got %q, want %q", got, newFile)
+		}
+	})
+}