Quickstart: verifying frozen lanes for started work packages

Reproduce #5573 (base commit) and verify the fix

uv sync --frozen --all-extras && uv pip install -e .
# End-to-end reproduction through finalize-tasks (red on the base commit, green after the fix):
.venv/bin/python -m pytest tests/integration/test_refinalize_keeps_started_lanes.py -q
# Compute-level invariant + permutation sweep (fast):
.venv/bin/python -m pytest tests/lanes/test_frozen_lane_membership.py -q

By hand

1. Create a lanes mission with WP01 (a.py) and WP02 (b.py), then spec-kitty agent mission finalize-tasks --mission <m> --json. The result is lane-a = [WP01] and lane-b = [WP02]. 2. spec-kitty implement WP02 --mission <m> and commit work in the lane-b worktree. 3. Amend WP01: owned_files: [a.py, b.py], dependencies: [WP02]. Commit. 4. Re-run finalize-tasks --json. Expected: success, with one lane lane-b = [WP02, WP01]. 5. Also start WP01 before step 3, on lane-a. Expected: exit 1, error_code: LANE_MEMBERSHIP_FROZEN, reason: started_lanes_collapsed, and nothing written.