fix: replace in-memory ID counter with atomic database counter

Replace the in-memory nextID counter with an atomic database-backed
counter using the issue_counters table. This fixes race conditions
when multiple processes create issues concurrently.

Changes:
- Add issue_counters table with atomic INSERT...ON CONFLICT pattern
- Remove in-memory nextID field and sync.Mutex from SQLiteStorage
- Implement getNextIDForPrefix() for atomic ID generation
- Update CreateIssue() to use database counter instead of memory
- Update RemapCollisions() to use database counter for collision resolution
- Clean up old planning and bug documentation files

Fixes the multi-process ID generation race condition tested in
cmd/bd/race_test.go.

internal/storage/sqlite/collision.go

@@ -232,15 +232,16 @@ func RemapCollisions(ctx context.Context, s *SQLiteStorage, collisions []*Collis
 	for _, collision := range collisions {
 		oldID := collision.ID
 
-		// Allocate new ID
-		s.idMu.Lock()
+		// Allocate new ID using atomic counter
 		prefix, err := s.GetConfig(ctx, "issue_prefix")
 		if err != nil || prefix == "" {
 			prefix = "bd"
 		}
-		newID := fmt.Sprintf("%s-%d", prefix, s.nextID)
-		s.nextID++
-		s.idMu.Unlock()
+		nextID, err := s.getNextIDForPrefix(ctx, prefix)
+		if err != nil {
+			return nil, fmt.Errorf("failed to generate new ID for collision %s: %w", oldID, err)
+		}
+		newID := fmt.Sprintf("%s-%d", prefix, nextID)
 
 		// Record mapping
 		idMapping[oldID] = newID