Add plain text (txt) export format to the TUI /session:export command.
- Add Session.as_export_plain_text() domain method that produces a
human-readable plain text transcript without Markdown formatting
- Update TuiCommandRouter._session_export() to accept 'txt' as a valid
format, building the plain text content via as_export_plain_text()
- Update the invalid-format error message to include 'txt' as a valid
option alongside 'json' and 'md'
- Add BDD scenarios covering plain text export via TUI command and
domain model, including with-messages and no-messages cases
- Add corresponding step definitions for the new BDD scenarios
The plain text format uses a simple separator-based layout:
Session: <id>
Actor: <actor>
...
----------------------------------------
[0] USER (timestamp):
message content
----------------------------------------
ISSUES CLOSED: #3036
freemo