guardrails: plugin hardening wave 2 for MVP floor #13

Parent epic: #16

Problem

The plugin MVP in #4 proves the extension surface, but it does not yet cover enough of the high-value hook migration set to support an MVP claim by itself.

Deliverables

• expand plugin policy to cover high-priority fast-feedback hooks from the migration inventory
• add crash-safe state or logging expectations where needed for local runtime continuity
• document which plugin behaviors are part of the MVP floor versus later operational hardening

Candidate scope from the inventory

• post-lint-format
• block-version-downgrade
• context-budget-*
• mark-factcheck-done and reset-factcheck
• targeted review or fact-check runtime state where feasible without broad core patches

Acceptance

• the plugin covers the highest-value remaining fast-feedback policies needed for MVP
• behavior is scenario-tested or otherwise runtime-verified
• scope remains thin-distribution-first and upstream-friendly