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

skills/autonomous-ai-agents/hermes-agent/SKILL.md

@@ -336,7 +336,7 @@ The registry of record is `hermes_cli/commands.py` — every consumer
 ~/.hermes/config.yaml       Main configuration
 ~/.hermes/.env              API keys and secrets
 $HERMES_HOME/skills/        Installed skills
-~/.hermes/sessions/         Legacy session artifacts (no longer written; state.db is canonical)
+~/.hermes/sessions/         Gateway routing index, request dumps, *.jsonl transcripts (and optional per-session JSON snapshots when sessions.write_json_snapshots: true)
 ~/.hermes/state.db          Canonical session store (SQLite + FTS5)
 ~/.hermes/logs/             Gateway and error logs
 ~/.hermes/auth.json         OAuth tokens and credential pools