feat(sessions): opt-in per-session JSON snapshot writer

PR #29182 deleted the per-session JSON snapshot writer outright because
state.db is canonical and the snapshots had no in-tree consumer.  Some
users have external tooling that reads `~/.hermes/sessions/session_{sid}.json`
directly, so reintroduce the writer behind a config flag that defaults
to off.

- Add `sessions.write_json_snapshots` (default False) to DEFAULT_CONFIG
- Restore `AIAgent._save_session_log` + `_clean_session_content` as
  gated methods.  When the flag is off the call is a fast no-op; when
  on, the writer behaves as before (atomic write, truncation guard
  preserved, REASONING_SCRATCHPAD → think tag normalization)
- Re-derive the target path from `agent.session_id` on each call so
  `/branch` and `/compress` re-points happen automatically — no need
  to restore the explicit re-point bookkeeping at call sites
- Wire the single call site in `_persist_session` (the cleanup-on-exit
  hook).  Did NOT restore the 7 intra-turn calls the original PR deleted
  — those were redundant writes within the same turn that doubled disk
  I/O without adding any persistence guarantee `_persist_session` does
  not already provide
- Read the flag once at agent init via `load_config()`, cache as
  `agent._session_json_enabled`
- Update `TestNoSessionJsonSnapshot` → `TestSessionJsonSnapshotOptIn`
  to pin behavior: default off (no file), opt-in true (file written),
  no-op method on default agents, logs_dir retained unconditionally
- Update CONTRIBUTING.md and the bundled `hermes-agent` skill to
  document the flag and its default

agent/agent_init.py

@@ -901,8 +901,19 @@ def init_agent(
     hermes_home = get_hermes_home()
     agent.logs_dir = hermes_home / "sessions"
     agent.logs_dir.mkdir(parents=True, exist_ok=True)
-    # session_log_file removed — state.db is the canonical message store.
-    # logs_dir retained for request_dump_*.json (debug breadcrumb path).
+    # Per-session JSON snapshot writer (~/.hermes/sessions/session_{sid}.json)
+    # is opt-in via sessions.write_json_snapshots (default False).  state.db
+    # is canonical — the snapshot is only useful for external tooling that
+    # reads the JSON files directly.  See run_agent._save_session_log.
+    agent._session_json_enabled = False
+    try:
+        from hermes_cli.config import load_config as _load_sess_cfg
+        _sess_cfg = (_load_sess_cfg().get("sessions") or {})
+        agent._session_json_enabled = bool(_sess_cfg.get("write_json_snapshots", False))
+    except Exception:
+        pass
+    # logs_dir is retained unconditionally for request_dump_*.json (debug
+    # breadcrumb path written by agent_runtime_helpers.dump_api_request_debug).
     
     # Track conversation messages for session logging
     agent._session_messages: List[Dict[str, Any]] = []