Provide a durable turn-completion signal for resumed sessions

## Summary

When a client resumes or reconnects after missing the live event stream, there does not appear to be a documented way in the CLI/SDK to query whether the previous turn already completed successfully.

`session.idle` appears to be the canonical live "turn is done / session is now idle" signal, but because it is ephemeral and not persisted to `events.jsonl`, a reconnecting client that missed it has to infer completion from persisted artifacts and coarse session metadata.

## Current situation

A resumed client can currently combine:

- live events, **if** it was attached when they arrived,
- session metadata such as `ModifiedTime`, and
- persisted artifacts such as `~/.copilot/session-state//events.jsonl`.

However, none of the documented SDK APIs seem to provide a direct answer to: **did the previous turn already finish, or is it still in progress?**

For example:

- `GetStatusAsync()` exposes CLI/protocol status, not per-session turn state
- `ListSessionsAsync()` / `GetSessionMetadataAsync()` expose metadata, not turn completion state
- `assistant.turn_end` is persisted, but is not a reliable terminal signal because it can occur between tool rounds

## Why this matters

For restart/reconnect flows, clients need an authoritative answer to whether the last turn finished cleanly.

Without that, they have to resort to heuristics such as:

1. scanning the tail of `events.jsonl`,
2. checking file mtime / file growth, and
3. waiting on timeout/watchdog logic.

Those heuristics are inherently unreliable for long-running or tool-heavy turns and can produce both false positives and false negatives.

## Requested improvement

Any of the following would solve the underlying problem:

1. **Persist a durable completion marker to `events.jsonl`** — either by persisting the final `session.idle`, or by emitting a dedicated persisted event such as `session.turn_complete`
2. **Expose a documented session/turn status API** that a reconnecting client can call after resume
3. **Persist a final session status snapshot** that can be read after reconnect

The important point is not the exact mechanism; it is that resumed clients need some **authoritative, durable completion signal** instead of having to guess.

## Scope

- Affects the CLI event log under `~/.copilot/session-state//events.jsonl`
- Affects SDK/app clients that restore or reconnect sessions after restart, reconnect, or crash recovery

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**Related downstream tracking issue:** https://github.com/PureWeen/PolyPilot/issues/538