feat(tui): opt-in auto-resume of the most recent session (#17130)

* feat(tui): opt-in auto-resume of the most recent session

`hermes --tui` always forges a fresh session at startup unless the user
sets `HERMES_TUI_RESUME=<id>`.  Disconnects, terminal-window crashes,
and accidental Ctrl+D therefore lose every piece of in-flight context
even though `state.db` still has the full history a `/resume` away.

Add an opt-in path that mirrors classic CLI's `hermes -c` muscle
memory: when `display.tui_auto_resume_recent: true` is set in
`~/.hermes/config.yaml`, the TUI looks up the most recent human-facing
session and resumes it instead of starting fresh.  Default off so
existing users aren't surprised; explicit `HERMES_TUI_RESUME` always
wins.

Wires:

* New `session.most_recent` JSON-RPC in `tui_gateway/server.py` that
  returns the first non-`tool` row from `list_sessions_rich`, or
  `{"session_id": null}` when none.  Uses the same deny-list as
  `session.list` so sub-agent rows can't sneak in.
* `createGatewayEventHandler.handleReady` re-ordered: explicit
  `STARTUP_RESUME_ID` first (unchanged), then conditional auto-resume
  via `config.get full → display.tui_auto_resume_recent`, then the
  legacy `newSession()` fallback.  Failures of either RPC fall back
  to `newSession()` so the path is always finite.
* Default `display.tui_auto_resume_recent: False` added to
  `DEFAULT_CONFIG` in `hermes_cli/config.py` (no `_config_version`
  bump per AGENTS.md — deep-merge handles the additive key).

Tests:

* 4 new vitest cases in `createGatewayEventHandler.test.ts` cover
  every gate-and-fallback combination (env wins, config off, config
  on with hit, config on with miss).
* 3 new pytest cases for `session.most_recent` (denied row skip,
  tool-only → null, db-unavailable → null).

Validation:
  scripts/run_tests.sh tests/test_tui_gateway_server.py — 93/93.
  cd ui-tui && npm run type-check — clean; npm test --run — 393/393.

* review(copilot): fold session.most_recent errors into null + extend ConfigDisplayConfig

* review(copilot): cover RPC-rejection fallbacks in auto-resume tests

tests/test_tui_gateway_server.py

@@ -2654,3 +2654,70 @@ def test_prompt_submit_skips_auto_title_when_response_empty(monkeypatch):
         )
 
     mock_title.assert_not_called()
+
+
+# ── session.most_recent ──────────────────────────────────────────────
+
+
+def test_session_most_recent_returns_first_non_denied(monkeypatch):
+    """Drops `tool` rows like session.list does, returns the first hit."""
+
+    class _DB:
+        def list_sessions_rich(self, *, source=None, limit=200):
+            return [
+                {"id": "tool-1", "source": "tool", "title": "noise", "started_at": 100},
+                {"id": "tui-1", "source": "tui", "title": "real", "started_at": 99},
+            ]
+
+    monkeypatch.setattr(server, "_get_db", lambda: _DB())
+
+    resp = server.handle_request(
+        {"id": "1", "method": "session.most_recent", "params": {}}
+    )
+
+    assert resp["result"]["session_id"] == "tui-1"
+    assert resp["result"]["title"] == "real"
+    assert resp["result"]["source"] == "tui"
+
+
+def test_session_most_recent_returns_null_when_only_tool_rows(monkeypatch):
+    class _DB:
+        def list_sessions_rich(self, *, source=None, limit=200):
+            return [{"id": "tool-1", "source": "tool", "started_at": 1}]
+
+    monkeypatch.setattr(server, "_get_db", lambda: _DB())
+
+    resp = server.handle_request(
+        {"id": "1", "method": "session.most_recent", "params": {}}
+    )
+
+    assert resp["result"]["session_id"] is None
+
+
+def test_session_most_recent_folds_db_exception_into_null_result(monkeypatch):
+    """Per contract, errors are folded into the null-result shape so
+    callers don't have to special-case JSON-RPC error envelopes for
+    'no answer' (Copilot review on #17130)."""
+
+    class _BrokenDB:
+        def list_sessions_rich(self, *, source=None, limit=200):
+            raise RuntimeError("db locked")
+
+    monkeypatch.setattr(server, "_get_db", lambda: _BrokenDB())
+
+    resp = server.handle_request(
+        {"id": "1", "method": "session.most_recent", "params": {}}
+    )
+
+    assert "error" not in resp
+    assert resp["result"]["session_id"] is None
+
+
+def test_session_most_recent_handles_db_unavailable(monkeypatch):
+    monkeypatch.setattr(server, "_get_db", lambda: None)
+
+    resp = server.handle_request(
+        {"id": "1", "method": "session.most_recent", "params": {}}
+    )
+
+    assert resp["result"]["session_id"] is None