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.

tui_gateway/server.py

@@ -3702,6 +3702,12 @@ def _(rid, params: dict) -> dict:
             new_key,
             source="tui",
             model=_resolve_model(),
+            # Stable _branched_from marker so list_sessions_rich() keeps the
+            # branch visible in /resume and /sessions. The TUI branch leaves
+            # the parent live (no end_reason='branched'), so the legacy
+            # end_reason heuristic never matches it — the marker is the only
+            # thing that surfaces TUI branches. See issue #20856.
+            model_config={"_branched_from": old_key},
             parent_session_id=old_key,
             cwd=_session_cwd(session),
         )