self-review 2026-07-28: close ISE lifecycle observability gaps

- session_start now also posted to the HTTP Engram via ise_post: the
  local engram_node_full write never crossed to the observable stream
  (sync flows HTTP->soul only), so boots 5+ were invisible — last
  visible session_start was boot 4, two weeks ago
- graceful shutdown emits a final ISE with boot/pulse/uptime; a boot
  with no shutdown event now reliably signals a crash/SIGKILL
- empty /api/sync responses emit a sync_empty warn ISE instead of
  being silently skipped — unreachable engram no longer looks
  identical to quiet-but-healthy
- sync backflow prune reads ENGRAM_ISE_RETENTION_MS instead of
  duplicating the 48h magic number server.el already honors

soul.el

@@ -346,6 +346,12 @@ fn emit_session_start_event() -> Void {
         el_from_float(0.9), el_from_float(0.9), el_from_float(1.0),
         "Episodic", tags
     )
+    // ALSO post to the HTTP Engram stream via ise_post (2026-07-28 self-review):
+    // engram_node_full above writes only the soul's in-process store, and sync
+    // flows HTTP→soul, never the reverse — so session_start events for boots 5+
+    // silently vanished from the observable ISE stream (last visible: boot 4).
+    // ise_post falls back to a local tagged node if the HTTP Engram is down.
+    ise_post(payload)
     // Prune accumulated session-start events — keep the 10 most recent.
     // engram_search_json returns results in insertion order (oldest first), so
     // results[0..count-11] are the oldest; forgetting them leaves the newest 10.