Add agentic PR review loop example

[tomqwu]

Summary:

• add a practical builder/reviewer PR loop knowledge note
• link the example from the AI-agent coding strategy and knowledge index

Changed files:

• docs/knowledge/agentic-pr-review-loop.md: documents the Claude-style builder and Codex-style reviewer prompts, scheduler, lock/state guards, and anti-patterns
• docs/knowledge/README.md: lists the new note
• docs/ai-agent-coding-strategy.md: cross-links the practical example from the strategy doc

Validation:

• ./scripts/check.sh

Follow-ups:

• none