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/index/tfidf.go

@@ -123,7 +123,12 @@ func (idx *TFIDFIndex) TopTerms(docIdx int, n int) []struct {
 			}{t, s})
 		}
 	}
-	sort.Slice(scored, func(i, j int) bool { return scored[i].Score > scored[j].Score })
+	sort.Slice(scored, func(i, j int) bool {
+		if scored[i].Score != scored[j].Score {
+			return scored[i].Score > scored[j].Score
+		}
+		return scored[i].Term < scored[j].Term
+	})
 	if len(scored) > n {
 		scored = scored[:n]
 	}
@@ -220,7 +225,12 @@ func (idx *TFIDFIndex) Cluster(minSharedTerms int, maxTopics int) []TopicCluster
 			for k, v := range allTerms {
 				sorted = append(sorted, kv{k, v})
 			}
-			sort.Slice(sorted, func(i, j int) bool { return sorted[i].v > sorted[j].v })
+			sort.Slice(sorted, func(i, j int) bool {
+				if sorted[i].v != sorted[j].v {
+					return sorted[i].v > sorted[j].v
+				}
+				return sorted[i].k < sorted[j].k
+			})
 
 			var nameParts []string
 			for _, kv := range sorted {