Display session ID when ending an interactive session

[crmitchelmore]

Describe the feature or problem you'd like to solve

When ending an interactive session (via /exit, Ctrl+C, or Ctrl+D), the session ID is not displayed. This makes it difficult to resume specific sessions later using copilot --resume <session-id>. Currently, the copilot --resume picker shows session descriptions, but these are often too vague to identify which session you want to resume - especially when restarting to reload configuration or recover from buggy behavior.

Proposed solution

Display the session ID in the session summary when ending an interactive session. For example:

Total usage est: 5 Premium requests Total duration (API): 45s Total duration (wall): 2m 30s Total code changes:
25 lines added, 10 lines removed
Session ID: 916524aa-24ae-406d-899a-698d2c19dd57 <-- ADD THIS

This complements #807 (which requests session ID in non-interactive -p mode) by covering the interactive mode exit flow.

Example prompts or workflows

1. User is in an interactive session working on a complex task
2. User needs to restart copilot (to reload config, work around a bug, etc.)
3. User types /exit or exits via ctrl+c and sees the session ID in the summary
4. User can immediately resume with copilot --resume <id>

Additional context

Related to #807 (session ID in non-interactive mode) and #820 (/resume slash command).

[chrys]

That will be very useful for "ralphing" also. +1