feat: Add @codebot explain-diff command for plain-language PR explanations

Implements code diff explainer that translates technical changes into
plain language for non-technical stakeholders (PMs, designers, new team members).

Features:
- Plain-language explanations without jargon
- File-by-file breakdown with 'what' and 'why' context
- Architecture impact analysis
- Breaking change detection
- Perfect for onboarding and cross-functional reviews

Implementation:
- Added explain_diff.md prompt template with plain-language guidelines
- Implemented _handle_explain_diff_command() in PRAgent
- Added _format_diff_explanation() for readable markdown
- Updated PRAgent.can_handle() to route explain-diff commands
- Added 'explain-diff' to config.yml commands list

Workflow Safety (prevents duplicate runs):
- Added '@codebot explain-diff' to ai-comment-reply.yml conditions
- Excluded from ai-chat.yml to prevent duplication
- Only triggers on PR comments (not issues)
- Manual command only (no automatic triggering)

Testing:
- 9 comprehensive tests in TestDiffExplanation class
- Tests command detection, formatting, plain-language output
- Verifies prompt formatting and empty section handling

Documentation:
- Updated README.md with explain-diff command and examples
- Added detailed implementation guide in CLAUDE.md
- Included plain-language rules and use cases

Related: Milestone 2 high-priority feature - code diff explainer

tools/ai-review/config.yml

@@ -68,6 +68,7 @@ interaction:
         - security
         - summarize # Generate PR summary (works on both issues and PRs)
         - changelog # Generate Keep a Changelog format entries (PR comments only)
+        - explain-diff # Explain code changes in plain language (PR comments only)
         - triage
         - review-again