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_test.go

@@ -485,13 +485,14 @@ func TestMigrateContentHashColumn(t *testing.T) {
 				original_type TEXT DEFAULT '',
 				sender TEXT DEFAULT '',
 				ephemeral INTEGER DEFAULT 0,
+				pinned INTEGER DEFAULT 0,
 				replies_to TEXT DEFAULT '',
 				relates_to TEXT DEFAULT '',
 				duplicate_of TEXT DEFAULT '',
 				superseded_by TEXT DEFAULT '',
 				CHECK ((status = 'closed') = (closed_at IS NOT NULL))
 			);
-			INSERT INTO issues SELECT id, title, description, design, acceptance_criteria, notes, status, priority, issue_type, assignee, estimated_minutes, created_at, updated_at, closed_at, external_ref, compaction_level, compacted_at, original_size, compacted_at_commit, source_repo, '', NULL, '', '', '', '', 0, '', '', '', '' FROM issues_backup;
+			INSERT INTO issues SELECT id, title, description, design, acceptance_criteria, notes, status, priority, issue_type, assignee, estimated_minutes, created_at, updated_at, closed_at, external_ref, compaction_level, compacted_at, original_size, compacted_at_commit, source_repo, '', NULL, '', '', '', '', 0, 0, '', '', '', '' FROM issues_backup;
 			DROP TABLE issues_backup;
 		`)
 		if err != nil {