Fix stale Crane completion recovery

[mrjf]

fix(crane): recover stale completed-label migrations

TL;DR

This fixes the scheduler path that let issue #78 stay marked complete even after the new deterministic Go cutover gate landed. The scheduler now treats crane-completed plus Completed: true as trustworthy only when it can positively confirm the current PR-head gate; no open migration PR, pending checks, failing checks, or unavailable check evidence makes the completion stale and selects the migration again.

Problem

• Issue Crane Migration: Python to Go -- Full APM CLI Rewrite #78 had crane-completed and repo-memory Completed: true, but there was no open migration PR to validate.
• The scheduler logged “no open migration PR was found” but did not mark the completion stale, so check_skip_conditions could still skip it as completed.
• The real repo-memory heading is ## [*] Machine State; the parser treated [*] as a regex character class and could parse that file as empty in local reproduction.

Approach

Area	Change
State parsing	Parse the literal ## [*] Machine State heading and skip markdown separator rows.
Stale recovery	Add evaluate_completed_label_recovery so completed-label issues require an affirmative PR-head gate.
Scheduler selection	Treat no PR or unknown/failing gate as stale, add the migration to stale_completed_state, and keep it due.
Tests	Add regression coverage for the real heading, no-open-PR stale recovery, and unknown-gate stale recovery.

Validation

.venv/bin/python -m pytest tests/unit/test_crane_scheduler.py -q
..........                                                               [100%]
10 passed in 0.43s

.venv/bin/python -m pytest tests/unit/test_crane_scheduler.py tests/unit/test_crane_workflow_prompt.py -q
...............                                                          [100%]
15 passed in 0.39s

python3 -m py_compile .github/workflows/scripts/crane_scheduler.py
git diff --check

Production-shaped local simulation against issue #78 and the current memory/crane state now emits:

{
  "selected": "crane-migration-python-to-go-full-apm-cli-rewrite",
  "selected_issue": 78,
  "stale_completed_state": [
    "crane-migration-python-to-go-full-apm-cli-rewrite"
  ],
  "skipped": [],
  "existing_pr": null
}

How to test

1. Run pytest tests/unit/test_crane_scheduler.py -q.
2. With Crane Migration: Python to Go -- Full APM CLI Rewrite #78 still labeled crane-completed and no open migration PR, run the scheduler with GITHUB_REPOSITORY=githubnext/apm.
3. Confirm /tmp/gh-aw/crane.json contains the migration in stale_completed_state and does not list it under skipped.

Trade-offs

• This errs on the side of reopening work when completion cannot be verified. That is intentional for Crane: missing deterministic evidence must not count as done.
• I did not change workflow markdown, so gh aw compile is not required for this PR.