Document runtime diagnostics proof rules for blocked local browser paths

Summary

The current runtime diagnostics blocker exposed a missing repo rule for how diagnostics-related UI proof should be captured, linked, and escalated when browser automation is unavailable.

Evidence

• Runtime diagnostics blocker issue #169 and PR #170 rely on an explicit proof gap because browser access to https://development.wp.local/ returns a Studio/Internal Server Error body outside studio wp.
• The repo has no durable guidance for when CI Playwright artifacts, Studio screenshots, or CLI-only evidence are acceptable substitutes.

Scope

• Define runtime-diagnostics proof rules for issues and PRs.
• Specify proof-source priority for local browser screenshots, Studio parity, CI Playwright artifacts, and CLI evidence.
• Require explicit blocker notes when browser proof cannot be captured locally.
• Clarify how diagnostics-specific regressions should be documented without stalling unrelated release work.

Acceptance criteria

• The repo has a documented proof rule for runtime diagnostics UI work.
• Contributors know what evidence is required before and after a diagnostics change.
• Browser-path blockers must be recorded explicitly with the failed environment and fallback proof source.
• Issue/PR threads can link to one canonical repo policy instead of re-explaining the proof rules each time.