fix(state): keep /branch sessions visible after parent reopen

/branch (aka /fork) sessions vanished from /resume and /sessions. Both
surfaces funnel through list_sessions_rich(include_children=False), which
hid any session with a parent_session_id unless identified as a branch via a
heuristic — parent.end_reason == 'branched' AND child.started_at >=
parent.ended_at.

Two ways that heuristic failed:
1. CLI/gateway branches: once the parent was reopened (e.g. resumed) and
   re-ended with a different end_reason (tui_shutdown overwriting 'branched'),
   the heuristic stopped matching and the branch was hidden permanently.
2. TUI branches (tui_gateway session.branch): the TUI never ends the parent
   as 'branched' — it creates the child while the parent is still live — so
   the heuristic NEVER matched and TUI branches were hidden from the moment
   they were created (this is the macOS desktop app's primary symptom).

Fix: persist a stable '_branched_from' marker in the branch session's
model_config at creation time across ALL THREE branch paths (CLI cli.py,
gateway gateway/run.py, and TUI tui_gateway/server.py), and OR a
json_extract(model_config, '$._branched_from') IS NOT NULL check into the
list_sessions_rich filter. The marker is immutable across the parent's
lifecycle, so the branch stays visible regardless of how/whether the parent
is ended. The legacy end_reason heuristic is kept (OR'd) so pre-existing
branches remain visible. Subagent/compression children (no marker, parent
not 'branched') stay correctly hidden. Fixes #20856.

Approach by liuhao1024 (PR #20864); reimplemented on current main, extended
to the TUI branch path (which the original missed), with regression tests for
the reopen+re-end scenario and the TUI marker persistence.

cli.py

@@ -7166,7 +7166,11 @@ class HermesCLI:
         except Exception:
             pass
 
-        # Create the new session with parent link
+        # Create the new session with parent link.
+        # Persist a stable ``_branched_from`` marker in model_config so
+        # list_sessions_rich() can keep the branch visible in /resume and
+        # /sessions even after the parent is reopened and re-ended with a
+        # different end_reason (e.g. tui_shutdown overwriting 'branched').
         try:
             self._session_db.create_session(
                 session_id=new_session_id,
@@ -7175,6 +7179,7 @@ class HermesCLI:
                 model_config={
                     "max_iterations": self.max_turns,
                     "reasoning_config": self.reasoning_config,
+                    "_branched_from": parent_session_id,
                 },
                 parent_session_id=parent_session_id,
             )