feat(sessions): let sessions_send register explicit state-change watchers (#104815)

* feat(sessions): let sessions_send register explicit state-change watchers

* feat(sessions): terse self-contained notices, immediate stale-state wakes, session-state docs page

* chore: regenerate prompt snapshots after rebase

docs/docs_map.md

@@ -2868,6 +2868,17 @@ Do not edit it by hand; run `pnpm docs:map:gen`.
   - H2: Index lifecycle
   - H2: Session search vs. memory search
 
+## concepts/session-state.md
+
+- Route: /concepts/session-state
+- Headings:
+  - H2: The signal log
+  - H2: Watchers
+  - H2: Notices: one, not many
+  - H2: Reconciling
+  - H2: Storage and limits
+  - H2: Related
+
 ## concepts/session-tool.md
 
 - Route: /concepts/session-tool