Enable errcheck linter and fix all production code warnings

- Enabled errcheck linter (previously disabled)
- Set tests: false in .golangci.yml to focus on production code
- Fixed 27 errcheck warnings using Go best practices:
  * Database resources: defer func() { _ = rows.Close() }()
  * Transaction rollbacks: defer func() { _ = tx.Rollback() }()
  * Best-effort closers: _ = store.Close(), _ = client.Close()
  * File writes: proper error checking on Close()
  * Interactive input: handle EOF gracefully
  * File ops: ignore ENOENT on os.Remove()
- All tests pass
- Closes bd-58

Amp-Thread-ID: https://ampcode.com/threads/T-57c9afd3-9adf-40c2-8be7-3e493d200361
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/compact.go

@@ -93,7 +93,7 @@ func (s *SQLiteStorage) GetTier1Candidates(ctx context.Context) ([]*CompactionCa
 	if err != nil {
 		return nil, fmt.Errorf("failed to query tier1 candidates: %w", err)
 	}
-	defer rows.Close()
+	defer func() { _ = rows.Close() }()
 
 	var candidates []*CompactionCandidate
 	for rows.Next() {
@@ -170,7 +170,7 @@ func (s *SQLiteStorage) GetTier2Candidates(ctx context.Context) ([]*CompactionCa
 	if err != nil {
 		return nil, fmt.Errorf("failed to query tier2 candidates: %w", err)
 	}
-	defer rows.Close()
+	defer func() { _ = rows.Close() }()
 
 	var candidates []*CompactionCandidate
 	for rows.Next() {
@@ -271,7 +271,7 @@ func (s *SQLiteStorage) ApplyCompaction(ctx context.Context, issueID string, lev
 	if err != nil {
 		return fmt.Errorf("failed to begin transaction: %w", err)
 	}
-	defer tx.Rollback()
+	defer func() { _ = tx.Rollback() }()
 	
 	var commitHashPtr *string
 	if commitHash != "" {