Add PR review gate to agent templates

[tomqwu]

Summary:

• add compact builder/reviewer PR review gate prompts to AGENTS.md and CLAUDE.md
• add the approval marker rule to the Copilot mirror so bootstrap files stay aligned

Changed files:

• AGENTS.md: adds the generic builder and reviewer PR-gate prompt snippets
• CLAUDE.md: adds the same builder/reviewer split with a link to the full knowledge note
• .github/copilot-instructions.md: mirrors the head-SHA approval gate for Copilot

Validation:

• ./scripts/check.sh

Follow-ups:

• none