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.

tests/test_hermes_state.py

@@ -2716,6 +2716,44 @@ class TestListSessionsRich:
         ids = [s["id"] for s in sessions]
         assert "branch" in ids, "Branch session should be visible in default list"
 
+    def test_branch_session_visible_after_parent_reopen_and_reend(self, db):
+        """Branch sessions stay visible after the parent is reopened and re-ended.
+
+        Regression for issue #20856: /branch (aka /fork) sessions vanished from
+        /resume and /sessions once the parent was reopened (e.g. resumed) and
+        re-ended with a different end_reason — tui_shutdown overwriting
+        'branched' — which broke the legacy end_reason heuristic. The stable
+        _branched_from marker in model_config keeps them visible.
+        """
+        import json as _json
+
+        db.create_session("parent", "cli")
+        db.end_session("parent", "branched")
+        db.create_session(
+            "branch",
+            "cli",
+            model_config={"_branched_from": "parent"},
+            parent_session_id="parent",
+        )
+        db.append_message("branch", "user", "Exploring the alternative approach")
+
+        # Marker is persisted at creation time.
+        branch_row = db.get_session("branch")
+        cfg = _json.loads(branch_row["model_config"]) if branch_row["model_config"] else {}
+        assert cfg.get("_branched_from") == "parent"
+
+        # Visible immediately after branching.
+        assert "branch" in [s["id"] for s in db.list_sessions_rich()]
+
+        # Parent reopened + re-ended with a different reason (the bug trigger).
+        db.reopen_session("parent")
+        db.end_session("parent", "tui_shutdown")
+
+        # Branch must STILL be visible — the marker survives the parent's
+        # end_reason churn, unlike the legacy 'branched' heuristic.
+        ids = [s["id"] for s in db.list_sessions_rich()]
+        assert "branch" in ids, "Branch should stay visible after parent re-end"
+
     def test_subagent_session_still_hidden(self, db):
         """Sub-agent children (parent NOT ended with 'branched') remain hidden."""
         db.create_session("root", "cli")