The 'sessions' command has been registered in the central command
registry since #20805 (May 2025) and surfaces in /help and tab-completion,
but the classic CLI's process_command() never had an elif branch for it.
The canonical name fell through and printed 'Unknown command: sessions'.
The TUI side was wired up correctly via the SessionPicker overlay; only
the legacy CLI was missing the dispatch.
Adds _handle_sessions_command() which mirrors /resume's no-arg behavior
inline (the CLI has no overlay primitive equivalent to the TUI picker):
- /sessions and /sessions list → print the recent-sessions table
- /sessions <id_or_title> → delegates to _handle_resume_command
Includes regression tests covering the dispatcher wiring (the original
bug) plus the three handler branches.
d6c488f2dc