feat(cleanup): protect pinned issues from cleanup/compact (bd-b2k)

Add Pinned field to Issue struct and database schema to protect
issues from accidental deletion via cleanup or compaction.

Changes:
- Add Pinned bool field to types.Issue
- Create migration 023_pinned_column.go for database schema
- Filter out pinned issues in cleanup command before deletion
- Add pinned check to GetTier1Candidates and GetTier2Candidates
- Add pinned check to CheckEligibility for compaction
- Update all SQL queries and scan functions to include pinned field

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

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

internal/storage/sqlite/migrations/023_pinned_column.go

@@ -0,0 +1,37 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+// MigratePinnedColumn adds the pinned column to the issues table.
+// Pinned issues are protected from cleanup and compaction operations (bd-b2k).
+func MigratePinnedColumn(db *sql.DB) error {
+	var columnExists bool
+	err := db.QueryRow(`
+		SELECT COUNT(*) > 0
+		FROM pragma_table_info('issues')
+		WHERE name = 'pinned'
+	`).Scan(&columnExists)
+	if err != nil {
+		return fmt.Errorf("failed to check pinned column: %w", err)
+	}
+
+	if columnExists {
+		return nil
+	}
+
+	_, err = db.Exec(`ALTER TABLE issues ADD COLUMN pinned INTEGER DEFAULT 0`)
+	if err != nil {
+		return fmt.Errorf("failed to add pinned column: %w", err)
+	}
+
+	// Add index for pinned issues (for efficient queries)
+	_, err = db.Exec(`CREATE INDEX IF NOT EXISTS idx_issues_pinned ON issues(pinned) WHERE pinned = 1`)
+	if err != nil {
+		return fmt.Errorf("failed to create pinned index: %w", err)
+	}
+
+	return nil
+}