feat: M1 determinism fixes for gossip

Refs #1

- F1: canonical fingerprints (git identity by remote URL, log by
  basename, obsidian by relative vault path) so identical facts get
  identical ids across machines
- F2: node_id (persisted in settings) + Hybrid Logical Clock in all
  observation "when" columns; removed time.Now() fact-time fallbacks
- F3: stable TF-IDF tie-break sort (score desc, term asc)
- F4: observations carry (node_id, hcl) locator; dedup ordered by hcl

internal/db/schema.go

@@ -18,7 +18,10 @@ CREATE TABLE IF NOT EXISTS observations (
     confidence       REAL DEFAULT 0.5,
     ingester_version TEXT DEFAULT '1',
     trigger          TEXT,
-    provenance       TEXT DEFAULT '{}'
+    provenance       TEXT DEFAULT '{}',
+    node_id          TEXT NOT NULL DEFAULT '',
+    hcl              INTEGER,
+    received_at      TEXT
 );
 
 CREATE INDEX IF NOT EXISTS idx_obs_fingerprint ON observations(fingerprint);