Remove the working directory when a session closes during setup

Closing an Atomic session while its first turn was still setting up ran
cleanup before the working directory existed; setup then created it,
saw the session closed, and stopped without deleting it. Delete the
directory on that closed path, in a finally so a failed dispose still
removes it.

Reported by Maggie Appleton in review.

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-verification: fail-before/pass-after passed: the new lifecycle test left a chopin-atomic-planner directory behind without the fix and passes with it
Assistant-verification: bun test passed: 1824 pass, 2 PostgreSQL skips, 0 fail
Assistant-verification: bun run types passed: all workspaces
Assistant-verification: bun run ci passed: dprint, oxlint, tokens, design contract, design record, Impeccable (no new findings)
Co-authored-by: Alex Lavaee <lavaman131@github.com>