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/ingest/gitea.go

@@ -148,14 +148,16 @@ func (g *GiteaIngester) fetchPulls() ([]teaPull, error) {
 	return pulls, nil
 }
 
-// signalTime returns the item's last-activity time, falling back to now.
+// signalTime returns the item's last-activity time, or "" when unknown. An
+// empty value is an explicit unknown — downstream uses it as a missing signal,
+// never fabricates a "now" timestamp that would differ across nodes.
 func signalTime(updated string) string {
 	if updated != "" {
 		if _, err := time.Parse(time.RFC3339, updated); err == nil {
 			return updated
 		}
 	}
-	return time.Now().UTC().Format(time.RFC3339)
+	return ""
 }
 
 func (g *GiteaIngester) repoToResult(r teaRepo) *IngestResult {