[Suggestion] Add persistent per-turn telemetry #34

Is your feature request related to a problem? Please describe.
The plugin aggregates session totals but does not retain a durable per-request ledger for timing, tool activity, retries, or failures.

Describe the solution you'd like
Persist a bounded, privacy-conscious turn record containing session/message IDs, model, token categories, cost, timestamps, finish status, retries, and tool summaries.

Additional context
Execution order: 2, after #33. Use a plugin-owned store and serialized writes without modifying OpenCode's database.