Reset mechanics

tilth reset [<session_id>] tears down a session's artifacts. It runs entirely outside the normal loop — no model calls, no validators, no evaluator. (The pre-Phase-3 --reset flag still works for one minor version.)

The flow

1. Resolve the session. Bare tilth reset picks the latest by directory name (parallel to tilth resume); explicit tilth reset <id> targets that one.
2. Recover paths. _read_checkpoint() gives workspace (worktree) and branch. _source_for_session() scans events.jsonl for the session_start event to recover the source repo (the path is already in the log, so no checkpoint schema change was needed for this).
3. Confirm. input("Continue? [y/N] ") unless --yes is passed. The prompt is the default; --yes is the override.
4. Tear down via ws.reset_session_state():
   • git worktree remove --force <worktree> against the source. Force is always passed: reset's whole purpose is to discard a session's work, and the user already confirmed via the [y/N] prompt (or --yes). Refusing on dirty would defeat the user's stated intent. A failure here now indicates a true filesystem-level problem (locks, perms) rather than uncommitted changes.
   • git branch -D session/<id> against the source (force-delete is correct for the session/* namespace, which is never auto-merged).
   • shutil.rmtree(sessions/<id>/) for whatever's left on the harness side.
5. Each step is idempotent — already-missing pieces are reported as skipped, not errored. You can run tilth reset against a half-cleaned-up state and it'll finish the job.

Diagram suggestion — step-by-step flow diagram of the four side effects, each with a green "ok" branch and a yellow "already gone, skipped" branch. Reinforces idempotency as a property of the whole command, not just one step.

There's no --reset --all and no "keep events" mode. The [y/N] prompt is the only safety gate; once you confirm, the session is gone.