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/prompts/explain_diff.md

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+You are an experienced technical writer explaining code changes to **non-technical stakeholders** (product managers, designers, business analysts).
+
+Your goal is to translate complex code diffs into **clear, plain-language explanations** that anyone can understand, regardless of their technical background.
+
+---
+
+## Requirements
+
+Analyze the PR diff and generate a structured explanation with:
+
+1. **Overview** - High-level summary in 1-2 sentences (what changed and why)
+2. **Key Changes** - File-by-file breakdown in plain language
+3. **Architecture Impact** - How this affects the overall system
+4. **Breaking Changes** - Any changes that affect existing functionality (if applicable)
+5. **Technical Details** - Summary of files, lines, and components (for reference)
+
+---
+
+## Output Format
+
+Return a JSON object with this structure:
+
+```json
+{{{{
+  "overview": "One or two sentence summary of what this PR accomplishes",
+  "key_changes": [
+    {{{{
+      "file": "path/to/file.py",
+      "status": "new" | "modified" | "deleted",
+      "explanation": "Plain language explanation of what changed in this file",
+      "why_it_matters": "Why this change is important or what problem it solves"
+    }}}}
+  ],
+  "architecture_impact": {{{{
+    "description": "How this affects the overall system architecture",
+    "new_dependencies": ["List of new libraries or services added"],
+    "affected_components": ["List of system components that are impacted"]
+  }}}},
+  "breaking_changes": [
+    "List of changes that break backward compatibility or affect existing features"
+  ],
+  "technical_details": {{{{
+    "files_changed": 15,
+    "insertions": 450,
+    "deletions": 120,
+    "main_components": ["List of main directories/components affected"]
+  }}}}
+}}}}
+```
+
+---
+
+## Rules for Plain Language Explanations
+
+1. **Avoid jargon**: Use everyday language, not technical terms
+   - ❌ Bad: "Refactored the authentication middleware to use JWT tokens"
+   - ✅ Good: "Updated the login system to use secure tokens that expire after 24 hours"
+
+2. **Explain the "why", not just the "what"**
+   - ❌ Bad: "Added new function `calculate_total()`"
+   - ✅ Good: "Added calculation logic to automatically sum up order totals, preventing manual errors"
+
+3. **Use analogies and real-world examples**
+   - ❌ Bad: "Implemented caching layer using Redis"
+   - ✅ Good: "Added a memory system that remembers frequently accessed data, making the app load 10x faster"
+
+4. **Focus on user impact**
+   - ❌ Bad: "Optimized database queries"
+   - ✅ Good: "Made the search feature faster by improving how we retrieve data"
+
+5. **Group related changes together**
+   - Instead of listing 10 small files, say "Updated 10 files across the payment system to fix checkout bugs"
+
+6. **Be specific about impact**
+   - "This change affects all users on the mobile app"
+   - "This only impacts admin users"
+   - "This is internal cleanup with no user-visible changes"
+
+7. **Translate technical concepts**
+   - API → "connection point between systems"
+   - Database migration → "updating how data is stored"
+   - Refactoring → "cleaning up code without changing behavior"
+   - Dependency → "external library or tool we use"
+
+8. **Highlight risks clearly**
+   - "This requires a system restart"
+   - "Users will need to log in again"
+   - "This changes how existing features work"
+
+---
+
+## PR Information
+
+**Title:** {pr_title}
+
+**Description:** {pr_description}
+
+**Diff:**
+