sessions.updated_at not refreshed as new turns are added

### Describe the bug

The `sessions.updated_at` field in the session store is set once (at session creation or first checkpoint) and never updated as new conversation turns are added. This causes long-running sessions to appear stale in queries, making them difficult to find when searching for recent work.

### Affected version

Copilot CLI 1.0.10-1

### Steps to reproduce the behavior

1. Start a new session and note the `sessions.updated_at` value.
2. Have a multi-turn conversation (5+ turns) over a period of time.
3. Query `sessions.updated_at` — it will still reflect the original value, not the latest turn.
4. Optionally run `/compact` and re-check — `updated_at` may still not refresh.

### Expected behavior

`sessions.updated_at` should reflect the timestamp of the most recent turn (or at minimum, the most recent checkpoint). This field is the natural "last active" indicator and is the first thing queried when searching for recent sessions.

### Additional context

Copilot-generated issue details: [session-store-updated-at-bug.md](https://github.com/user-attachments/files/26146946/session-store-updated-at-bug.md)