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

CLAUDE.md

@@ -217,6 +217,7 @@ Prompts are stored in `tools/ai-review/prompts/` as Markdown files:
 - `base.md` - Base instructions for all reviews
 - `pr_summary.md` - PR summary generation template
 - `changelog.md` - Keep a Changelog format generation template
+- `explain_diff.md` - Plain-language diff explanation template
 - `issue_triage.md` - Issue classification template
 - `issue_response.md` - Issue response template
 
@@ -533,6 +534,79 @@ The `@codebot changelog` command generates Keep a Changelog format entries from
 - Excluded from ai-chat.yml to prevent duplicate runs
 - No automatic triggering - manual command only
 
+### Code Diff Explainer
+
+The `@codebot explain-diff` command translates technical code changes into plain language for non-technical stakeholders.
+
+**Key Features:**
+- Plain-language explanations without jargon
+- File-by-file breakdown with "what" and "why" context
+- Architecture impact analysis
+- Breaking change detection
+- Perfect for PMs, designers, and new team members
+
+**Implementation Details:**
+
+1. **Command Handler** - `PRAgent._handle_explain_diff_command()`:
+   - Triggered by `@codebot explain-diff` in PR comments
+   - Fetches PR title, description, and full diff
+   - Loads `prompts/explain_diff.md` template
+   - Formats prompt with PR context
+
+2. **LLM Analysis** - Generates plain-language JSON:
+   ```json
+   {
+     "overview": "High-level summary in everyday language",
+     "key_changes": [
+       {
+         "file": "path/to/file.py",
+         "status": "new|modified|deleted",
+         "explanation": "What changed (no jargon)",
+         "why_it_matters": "Business/user impact"
+       }
+     ],
+     "architecture_impact": {
+       "description": "System-level effects explained simply",
+       "new_dependencies": ["External libraries added"],
+       "affected_components": ["System parts impacted"]
+     },
+     "breaking_changes": ["User-facing breaking changes"],
+     "technical_details": { /* Stats for reference */ }
+   }
+   ```
+
+3. **Formatting** - `_format_diff_explanation()`:
+   - Converts JSON to readable markdown
+   - Uses emojis for visual categorization (➕ new, 📝 modified, 🗑️ deleted)
+   - Highlights breaking changes prominently
+   - Includes technical summary for developers
+   - Omits empty sections for clean output
+
+4. **Prompt Engineering** - `prompts/explain_diff.md`:
+   - **Avoids jargon**: "API" → "connection point between systems"
+   - **Explains why**: Not just what changed, but why it matters
+   - **Uses analogies**: "Caching" → "memory system for faster loading"
+   - **Focus on impact**: Who is affected and how
+   - **Groups changes**: Combines related files into themes
+   - **Translates concepts**: Technical terms → everyday language
+
+**Plain Language Rules:**
+- ❌ "Refactored authentication middleware" → ✅ "Updated login system for better security"
+- ❌ "Implemented Redis caching" → ✅ "Added memory to make pages load 10x faster"
+- ❌ "Database migration" → ✅ "Updated how data is stored"
+
+**Common Use Cases:**
+- New team members understanding large PRs
+- Non-technical reviewers (PMs, designers) reviewing features
+- Documenting architectural decisions
+- Learning from other developers' code
+
+**Workflow Safety:**
+- Only triggers on PR comments (not issue comments)
+- Included in ai-comment-reply.yml workflow conditions
+- Excluded from ai-chat.yml to prevent duplicate runs
+- No automatic triggering - manual command only
+
 ### Review-Again Command Implementation
 
 The `@codebot review-again` command allows manual re-triggering of PR reviews without new commits.
@@ -591,6 +665,7 @@ Example commands:
 - `@codebot suggest` - Suggest solutions
 - `@codebot summarize` - Generate PR summary or issue summary (works on both)
 - `@codebot changelog` - Generate Keep a Changelog format entries (PR comments only)
+- `@codebot explain-diff` - Explain code changes in plain language (PR comments only)
 - `@codebot setup-labels` - Automatic label setup (built-in, not in config)
 - `@codebot review-again` - Re-run PR review without new commits (PR comments only)