feat: persist grep-friendly local thread history - #18

The CLI currently displays live threads but leaves no local copy to search later. This adds automatic, grep-friendly JSONL history at ~/.grok-bot-cli/history.jsonl: successful send commands record prompts, and thread/chat record fetched messages with full text, target IDs/names, roles, recording times, and available message/root IDs.

gbot history Researcher --search timeout
gbot history --search timeout --limit 100 --json
grep -in 'timeout' "$(gbot history --path)"

History commands run offline before backend/auth initialization. --history-dir / GROK_BOT_HISTORY_DIR configure storage; --no-history / GROK_BOT_HISTORY=off disable recording. New directories/files use 0700/0600 permissions, writes refuse file symlinks, and only conversation fields are saved. Disk failures warn on stderr without making a successful send look failed. Reads skip malformed records, and the next append separates an interrupted final record.

This records CLI observations: replies are saved when fetched, repeated fetches append repeated observations, and there is no background sync, deduplication, expiry, or session resumption. Conversation text is stored as plaintext; these limits and opt-outs are documented.

The shared transcript parser also handles array-shaped responses correctly and prefers full content over previews. Includes README/help updates and a minor changeset.

Validation: all 34 tests pass on Node 18, 22, and 24, including 10 new integration tests using a local mock gateway and isolated temporary homes. Coverage includes persistence across processes, real grep, bot/group filtering, multiline/long content, opt-outs, failed sends, disk errors, concurrent appends, and malformed history. Publint 0.3.24 and packed executable install/help/offline-history smoke checks pass. No live bot operations were used.