feat: M2 composite locator + idle reconcile

Refs #1

- observations: hcl backfilled from local rowid; (node_id, hcl) UNIQUE
  locator index becomes the gossip merge key
- threads: cluster_key column + partial UNIQUE index; CreateThreadCluster
  is idempotent, threader folds into exact cluster_key before heuristic
- AutoLinkThreadObservations dedup re-expressed on hcl DESC
- new `knox reconcile [--dry-run]` rebuilds entries from the observation
  log and re-links threads idempotently (entry count, thread cluster_key
  verified bit-identical from log-only DB)

main.go

@@ -63,6 +63,7 @@ and maintains a searchable index. Use 'knox watch' for daemon mode.`,
 	root.AddCommand(knoxcmd.NewTopicsCmd(kdb))
 	root.AddCommand(knoxcmd.NewGiteaCmd(kdb))
 	root.AddCommand(knoxcmd.NewGitCmd(kdb))
+	root.AddCommand(knoxcmd.NewReconcileCmd(kdb))
 	root.AddCommand(newServeCmd(kdb))
 
 	// MCP server subcommand