refactor: remove unused bd pin/unpin/hook commands (bd-x0zl)

Analysis found these commands are dead code:
- gt never calls `bd pin` - uses `bd update --status=pinned` instead
- Beads.Pin() wrapper exists but is never called
- bd hook functionality duplicated by gt mol status
- Code comment says "pinned field is cosmetic for bd hook visibility"

Removed:
- cmd/bd/pin.go
- cmd/bd/unpin.go
- cmd/bd/hook.go

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

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

cmd/bd/doctor/fix/fs_test.go

@@ -0,0 +1,71 @@
+package fix
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+)
+
+func TestMoveFile_EXDEV_FallsBackToCopy(t *testing.T) {
+	root := t.TempDir()
+	src := filepath.Join(root, "src.txt")
+	dst := filepath.Join(root, "dst.txt")
+	if err := os.WriteFile(src, []byte("hello"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	oldRename := renameFile
+	defer func() { renameFile = oldRename }()
+	renameFile = func(oldpath, newpath string) error {
+		return &os.LinkError{Op: "rename", Old: oldpath, New: newpath, Err: syscall.EXDEV}
+	}
+
+	if err := moveFile(src, dst); err != nil {
+		t.Fatalf("moveFile failed: %v", err)
+	}
+	if _, err := os.Stat(src); !os.IsNotExist(err) {
+		t.Fatalf("expected src to be removed, stat err=%v", err)
+	}
+	data, err := os.ReadFile(dst)
+	if err != nil {
+		t.Fatalf("read dst: %v", err)
+	}
+	if string(data) != "hello" {
+		t.Fatalf("dst contents=%q", string(data))
+	}
+}
+
+func TestMoveFile_EXDEV_CopyFails_LeavesSource(t *testing.T) {
+	root := t.TempDir()
+	src := filepath.Join(root, "src.txt")
+	dst := filepath.Join(root, "dst.txt")
+	if err := os.WriteFile(src, []byte("hello"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	oldRename := renameFile
+	oldOpenRW := openFileRW
+	defer func() {
+		renameFile = oldRename
+		openFileRW = oldOpenRW
+	}()
+	renameFile = func(oldpath, newpath string) error {
+		return &os.LinkError{Op: "rename", Old: oldpath, New: newpath, Err: syscall.EXDEV}
+	}
+	openFileRW = func(name string, flag int, perm os.FileMode) (*os.File, error) {
+		return nil, &os.PathError{Op: "open", Path: name, Err: syscall.ENOSPC}
+	}
+
+	err := moveFile(src, dst)
+	if err == nil {
+		t.Fatalf("expected error")
+	}
+	if !errors.Is(err, syscall.ENOSPC) {
+		t.Fatalf("expected ENOSPC, got %v", err)
+	}
+	if _, err := os.Stat(src); err != nil {
+		t.Fatalf("expected src to remain, stat err=%v", err)
+	}
+}