[codex] Include all open issues in stale reminders

[redreceipt]

Summary

• add a stale-reminder-specific Linear query that fetches open, non-project APO issues without label or priority constraints
• switch post_stale() to use that broader issue set while leaving priority/label dashboard queries unchanged
• add regression coverage for unprioritized, unlabeled stale issues reaching the Slack post path

Why

APO-7555 was old and open but missing from the stale Slack post because the stale job only queried Bug, Feature Request, and Technical Change issues with priority 1-5. The stale reminder should consider all open non-project issues, then apply the existing age and assignee filters.

Proof

• .venv-ci/bin/ruff check jobs.py linear/issues.py tests/test_jobs.py tests/test_linear_issue_state_filters.py
• .venv-ci/bin/python -m unittest tests.test_jobs tests.test_linear_issue_state_filters
• .venv-ci/bin/python -m unittest discover -s tests -p 'test_*.py'
• Live mocked-post readback confirmed APO-7555 appears in the generated stale message under dylan at 74d stale.