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
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