fix(storage): race condition when reconnect closes db mid-query (GH#607)

Change reconnectMu from sync.Mutex to sync.RWMutex so read operations
can hold RLock during database access. This prevents reconnect() from
closing the connection while queries are in progress.

- GetIssue and SearchIssues now hold RLock during database operations
- Close() acquires write lock to coordinate with reconnect
- Add TestConcurrentReadsWithReconnect to verify the fix

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

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

internal/storage/sqlite/queries.go

@@ -227,6 +227,11 @@ func (s *SQLiteStorage) GetIssue(ctx context.Context, id string) (*types.Issue,
 	// Check for external database file modifications (daemon mode)
 	s.checkFreshness()
 
+	// Hold read lock during database operations to prevent reconnect() from
+	// closing the connection mid-query (GH#607 race condition fix)
+	s.reconnectMu.RLock()
+	defer s.reconnectMu.RUnlock()
+
 	var issue types.Issue
 	var closedAt sql.NullTime
 	var estimatedMinutes sql.NullInt64
@@ -1400,6 +1405,11 @@ func (s *SQLiteStorage) SearchIssues(ctx context.Context, query string, filter t
 	// Check for external database file modifications (daemon mode)
 	s.checkFreshness()
 
+	// Hold read lock during database operations to prevent reconnect() from
+	// closing the connection mid-query (GH#607 race condition fix)
+	s.reconnectMu.RLock()
+	defer s.reconnectMu.RUnlock()
+
 	whereClauses := []string{}
 	args := []interface{}{}