feat: /undo command — remove last turn from the conversation

[Betterlol]

Summary

Adds /undo [count] to the TUI, letting the user remove the last N events from the durable session and rebuild the transcript.

Changes

Protocol layer (protocol/)

• Command::Undo(count~ : Int = 1) — wire format: {"command":"undo"} or {"command":"undo","count":N}
• Event::SessionUndone(events_removed~ : Int) — {"event":"session_undone","events_removed":N}
• Round-trip covered by the emit/parse test suite

Session model (agent_session/)

• Session::remove_last(count) — returns new session with last N events removed (immutable, uses @vector slicing)

Serve engine (cmd/openseek/serve.mbt)

• Undo dispatched in the serve loop: checks idle state, calls session.remove_last(count) + store.create(modified)
• Rejects with command_error when work is in flight or no durable session

TUI (cmd/tui/)

• /undo [count] slash command (default count=1, rejects oneshot mode)
• Ui::replace_transcript — scrollback-level rebuild for transcript rollback
• EngineSessionUndone — untagged message that survives stale-run guard, triggers session reload + transcript rebuild

Desktop (desktop/)

• Exhaustive decode matches updated for the new event variant

Verification

• moon check: clean (native + js)
• moon fmt --check: clean
• moon test: protocol 16, cmd/tui 83, agent_session 35, tui all pass
• moon info: regenerated .mbti files