Remove sequential ID code path (bd-aa744b)

- Removed nextSequentialID() and getIDMode() functions
- Removed issue_counters table from schema
- Made SyncAllCounters() a no-op for backward compatibility
- Simplified ID generation to hash-only (adaptive length)
- Removed id_mode config setting
- Removed sequential ID tests and migration code
- Updated CONFIG.md and AGENTS.md to remove sequential ID references

Follow-up bd-2a70 will remove obsolete test files and renumber command.

cmd/bd/migrate.go

@@ -357,22 +357,6 @@ This command:
 						color.Green("✓ Migrated %d issues to hash-based IDs\n", len(mapping))
 					}
 				}
-				
-				// Set id_mode=hash after successful migration (not in dry-run)
-				if !dryRun {
-					store, err := sqlite.New(targetPath)
-					if err == nil {
-						ctx := context.Background()
-						if err := store.SetConfig(ctx, "id_mode", "hash"); err != nil {
-							if !jsonOutput {
-								fmt.Fprintf(os.Stderr, "Warning: failed to set id_mode=hash: %v\n", err)
-							}
-						} else if !jsonOutput {
-							color.Green("✓ Switched database to hash ID mode\n")
-						}
-						store.Close()
-					}
-				}
 			} else {
 				store.Close()
 				if !jsonOutput {