Add Markdown export format for sessions alongside the existing JSON export.
Implement TUI slash command routing for /session:export and /session:import.
- Add Session.as_export_markdown() domain method producing a human-readable
Markdown transcript (lossy, not importable — for sharing/documentation)
- Extend CLI 'agents session export' with --format flag (json|md)
- Extend TuiCommandRouter._session_command() to handle 'export' and 'import'
subcommands, delegating to the session service via the DI container
- Add 16 Behave scenarios covering domain model, CLI, and TUI command paths
Closes#1004
freemo