Define E23-T1 PR review rubric and isolate runtime state

[dmoliveira]

Summary

• define Epic 23 Task 23.1 review rubric contract in instructions/pr_review_rubric.md with deterministic risk categories, severity/confidence scales, and conservative blocker evidence gates inspired by structured safety/triage patterns
• mark Epic 23 in progress and Task 23.1 complete in the roadmap, and document the baseline in README/changelog
• fix startup config breakage by moving plan_execution runtime persistence out of opencode.json into ~/.config/opencode/my_opencode/runtime/plan_execution.json, and update /start-work, /resume, /autoflow, /todo, /budget, digest, and selftests accordingly

Validation

• make validate
• make selftest
• make install-test