[Suggestion] Add automatic session receipts #42

Is your feature request related to a problem? Please describe.
Exports are manual and do not provide a concise end-of-session summary for sharing or reviewing.

Describe the solution you'd like
Generate an optional session receipt containing cost, tokens, models, latency, context health, tools, compactions, changes, and recommendations, with Markdown, JSON, and file export support.

Additional context
Execution order: 10, after #41. Coordinate with the existing dashboard backlog in #13 rather than duplicating it.