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)

docs/gossip-protocol.md

@@ -213,14 +213,17 @@ truth.
 
 ## 8. Milestones
 
-**M1 — Determinism fixes (prereq).** F1 (canonical fingerprints), F2 (HLC in all
-"when" columns), F3 (TF-IDF tie-break), F4 (locator IDs + `ORDER BY hcl`).
+**M1 — Determinism fixes (prereq).** DONE. F1 (canonical fingerprints), F2 (HLC
+in all "when" columns), F3 (TF-IDF tie-break), F4 (locator IDs + `ORDER BY hcl`).
 Verify: two fresh DBs ingesting the same real content produce identical
 observation hashes and identical threads (minus node_id).
 
-**M2 — Composite PK + reconcile.** Schema migration; `knox reconcile`; `entries`
-fully derived; dedup re-expressed on HCL. Verify: reconcile is idempotent; a DB
-with only the log reconstructs `entries`/threads bit-identical to the original.
+**M2 — Composite PK + reconcile.** DONE. HCL backfilled from local rowid;
+`(node_id, hcl)` locator UNIQUE index; threads carry `cluster_key` (partial
+UNIQUE index) making auto-creation idempotent; `knox reconcile` rebuilds
+`entries` from the log and re-links threads. Verify: reconcile is idempotent (0
+create/0 link on re-run); a log-only DB reconstructs `entries` and thread
+`cluster_key`s bit-identical to the original.
 
 **M3 — Peer protocol.** `peers` settings table, `/v1/ping`, `/v1/log` pull,
 `/v1/obs/batch` push, handshake + periodic anti-entropy, echo suppression.