feat(gateway): session activity watchdog, stall notify, compress timeout (#72424)

Three mechanisms to detect and notify when gateway sessions stall silently:

1. Mid-turn activity heartbeats stamped to SessionDB so hermes sessions list
   and hermes status show progress during long turns without new message rows.

2. Stall watchdog: when a busy session has pending inbound and the shared
   activity clock is idle past agent.session_stall_timeout (default 300),
   log a WARNING and notify the user once to try /new. Notify-only; does
   not kill the turn.

3. Compaction timeout: fenceless compress_context callers get a progress-aware
   host budget (compression.context_timeout_seconds default 120 idle,
   compression.context_total_ceiling_seconds default 600 ceiling). On timeout,
   cancel via commit fence, skip compaction without dropping messages, and
   continue the turn.

Closes #72016 (slices 1-3; slice 4 cumulative SSE stream-retry deadline
remains a follow-up).

Cherry-picked from PR #72424 by @fangliquanflq.

hermes_cli/dump.py

@@ -236,6 +236,7 @@ def _config_overrides(config: dict) -> dict[str, str]:
     interesting_paths = [
         ("agent", "max_turns"),
         ("agent", "gateway_timeout"),
+        ("agent", "session_stall_timeout"),
         ("agent", "tool_use_enforcement"),
         ("terminal", "backend"),
         ("terminal", "docker_image"),