feat(compact): Add --prune mode for standalone tombstone pruning (bd-c7y5)

Add a new --prune mode to `bd compact` that removes expired tombstones from
issues.jsonl without requiring AI compaction or deleting closed issues.

Features:
- `bd compact --prune` removes tombstones older than 30 days (default TTL)
- `bd compact --prune --older-than N` uses custom N-day TTL
- `bd compact --prune --dry-run` previews what would be pruned
- Supports --json output for programmatic use

This reduces sync overhead by eliminating accumulated tombstones that were
previously only pruned as a side effect of compaction or cleanup operations.

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

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

cmd/bd/compact_test.go

@@ -514,6 +514,189 @@ func TestPruneExpiredTombstones(t *testing.T) {
 	}
 }
 
+func TestPruneExpiredTombstones_CustomTTL(t *testing.T) {
+	// Setup: create a temp .beads directory with issues.jsonl
+	tmpDir := t.TempDir()
+	beadsDir := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		t.Fatalf("Failed to create .beads dir: %v", err)
+	}
+
+	issuesPath := filepath.Join(beadsDir, "issues.jsonl")
+	now := time.Now()
+
+	// Both tombstones are older than 5 days, so both should be pruned with 5-day TTL
+	tombstoneTime := now.Add(-10 * 24 * time.Hour) // 10 days ago
+
+	issues := []*types.Issue{
+		{
+			ID:        "test-live",
+			Title:     "Live issue",
+			Status:    types.StatusOpen,
+			Priority:  2,
+			IssueType: types.TypeTask,
+			CreatedAt: now.Add(-5 * 24 * time.Hour),
+			UpdatedAt: now,
+		},
+		{
+			ID:           "test-tombstone-1",
+			Title:        "(deleted)",
+			Status:       types.StatusTombstone,
+			Priority:     0,
+			IssueType:    types.TypeTask,
+			CreatedAt:    now.Add(-20 * 24 * time.Hour),
+			UpdatedAt:    tombstoneTime,
+			DeletedAt:    &tombstoneTime,
+			DeletedBy:    "alice",
+			DeleteReason: "duplicate",
+		},
+	}
+
+	// Write issues to JSONL
+	file, err := os.Create(issuesPath)
+	if err != nil {
+		t.Fatalf("Failed to create issues.jsonl: %v", err)
+	}
+	encoder := json.NewEncoder(file)
+	for _, issue := range issues {
+		if err := encoder.Encode(issue); err != nil {
+			file.Close()
+			t.Fatalf("Failed to write issue: %v", err)
+		}
+	}
+	file.Close()
+
+	// Save original dbPath and restore after test
+	originalDBPath := dbPath
+	defer func() { dbPath = originalDBPath }()
+	dbPath = filepath.Join(beadsDir, "beads.db")
+
+	// Run pruning with 5-day TTL - tombstone is 10 days old, should be pruned
+	customTTL := 5 * 24 * time.Hour
+	result, err := pruneExpiredTombstones(customTTL)
+	if err != nil {
+		t.Fatalf("pruneExpiredTombstones failed: %v", err)
+	}
+
+	// Verify results - 5-day TTL means tombstones older than 5 days are pruned
+	if result.PrunedCount != 1 {
+		t.Errorf("Expected 1 pruned tombstone with 5-day TTL, got %d", result.PrunedCount)
+	}
+	if result.TTLDays != 5 {
+		t.Errorf("Expected TTLDays 5, got %d", result.TTLDays)
+	}
+}
+
+func TestPreviewPruneTombstones(t *testing.T) {
+	// Setup: create a temp .beads directory with issues.jsonl
+	tmpDir := t.TempDir()
+	beadsDir := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		t.Fatalf("Failed to create .beads dir: %v", err)
+	}
+
+	issuesPath := filepath.Join(beadsDir, "issues.jsonl")
+	now := time.Now()
+
+	expiredTombstoneTime := now.Add(-60 * 24 * time.Hour) // 60 days ago
+
+	issues := []*types.Issue{
+		{
+			ID:        "test-live",
+			Title:     "Live issue",
+			Status:    types.StatusOpen,
+			Priority:  2,
+			IssueType: types.TypeTask,
+			CreatedAt: now,
+			UpdatedAt: now,
+		},
+		{
+			ID:           "test-expired-tombstone",
+			Title:        "(deleted)",
+			Status:       types.StatusTombstone,
+			Priority:     0,
+			IssueType:    types.TypeTask,
+			CreatedAt:    now.Add(-90 * 24 * time.Hour),
+			UpdatedAt:    expiredTombstoneTime,
+			DeletedAt:    &expiredTombstoneTime,
+			DeletedBy:    "bob",
+			DeleteReason: "obsolete",
+		},
+	}
+
+	// Write issues to JSONL
+	file, err := os.Create(issuesPath)
+	if err != nil {
+		t.Fatalf("Failed to create issues.jsonl: %v", err)
+	}
+	encoder := json.NewEncoder(file)
+	for _, issue := range issues {
+		if err := encoder.Encode(issue); err != nil {
+			file.Close()
+			t.Fatalf("Failed to write issue: %v", err)
+		}
+	}
+	file.Close()
+
+	// Save original dbPath and restore after test
+	originalDBPath := dbPath
+	defer func() { dbPath = originalDBPath }()
+	dbPath = filepath.Join(beadsDir, "beads.db")
+
+	// Preview pruning - should not modify file
+	result, err := previewPruneTombstones(0)
+	if err != nil {
+		t.Fatalf("previewPruneTombstones failed: %v", err)
+	}
+
+	// Verify preview results
+	if result.PrunedCount != 1 {
+		t.Errorf("Expected 1 tombstone to prune, got %d", result.PrunedCount)
+	}
+	if result.PrunedIDs[0] != "test-expired-tombstone" {
+		t.Errorf("Expected PrunedIDs [test-expired-tombstone], got %v", result.PrunedIDs)
+	}
+
+	// Verify file was NOT modified (preview mode)
+	file, err = os.Open(issuesPath)
+	if err != nil {
+		t.Fatalf("Failed to reopen issues.jsonl: %v", err)
+	}
+	defer file.Close()
+
+	var remaining []*types.Issue
+	decoder := json.NewDecoder(file)
+	for {
+		var issue types.Issue
+		if err := decoder.Decode(&issue); err != nil {
+			if err.Error() == "EOF" {
+				break
+			}
+			t.Fatalf("Failed to decode issue: %v", err)
+		}
+		remaining = append(remaining, &issue)
+	}
+
+	// Both issues should still be in file (preview doesn't modify)
+	if len(remaining) != 2 {
+		t.Errorf("Expected 2 issues (preview mode), got %d", len(remaining))
+	}
+}
+
+func TestCompactPruneFlagExists(t *testing.T) {
+	// Verify --prune flag exists
+	pruneFlag := compactCmd.Flags().Lookup("prune")
+	if pruneFlag == nil {
+		t.Error("compact command should have --prune flag")
+	}
+
+	// Verify --older-than flag exists
+	olderThanFlag := compactCmd.Flags().Lookup("older-than")
+	if olderThanFlag == nil {
+		t.Error("compact command should have --older-than flag")
+	}
+}
+
 func TestPruneExpiredTombstones_NoTombstones(t *testing.T) {
 	// Setup: create a temp .beads directory with only live issues
 	tmpDir := t.TempDir()