feat: persist unblock task planning artifacts

[xiaojiou176]

Summary

• add a first-class unblock_task.v1.json schema for L0-managed independent temporary unblock assignments
• derive unblock_tasks from worker continuation policy during intake preview and persist planning_unblock_tasks.json into run-local planning artifacts
• register the new artifact in run manifests and cover the schema/preview/persistence chain with targeted orchestrator tests

Why

• Unblock Task was already referenced by runtime policy and worker continuation strings, but it still lacked a real object contract and run-local artifact chain
• this PR turns that string-only policy hook into a previewable and persisted planning object without changing execution authority
• task_contract remains the only execution authority; unblock_tasks stays advisory planning/read-model data

Verification

• bash scripts/run_governance_py.sh scripts/validate_schemas.py
• PYTHONPATH=apps/orchestrator/src /Users/yuyifeng/.cache/cortexpilot/toolchains/python/current/bin/python -m pytest -q apps/orchestrator/tests/test_schema_validation.py apps/orchestrator/tests/test_intake_preview_helpers.py apps/orchestrator/tests/test_main_pm_intake_helpers_branches.py
• npm run truth:triage (repo-side green; external truth still red only on the known Docker/platform upstream records: ci-core-image.json and ui-audit-playwright.json)

Scope guard

• This wave intentionally stops at schema + preview + persisted artifact + manifest ref.
• It does not yet add a dedicated UI projection for unblock_tasks; that can be a follow-up once this object chain lands on main.