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)

internal/db/schema.go

@@ -101,9 +101,12 @@ CREATE TABLE IF NOT EXISTS threads (
     updated_at  TEXT DEFAULT (datetime('now')),
     resolved_at TEXT,
     tags        TEXT DEFAULT '[]',
-    provenance  TEXT DEFAULT '{}'
+    provenance  TEXT DEFAULT '{}',
+    cluster_key TEXT
 );
 
+CREATE UNIQUE INDEX IF NOT EXISTS idx_threads_cluster ON threads(cluster_key) WHERE cluster_key IS NOT NULL AND cluster_key != '';
+
 -- SETTINGS: key-value store for runtime configuration
 CREATE TABLE IF NOT EXISTS settings (
     key   TEXT PRIMARY KEY,