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

.gitea/workflows/ai-chat.yml

@@ -26,6 +26,7 @@ jobs:
             !contains(github.event.comment.body, '@codebot security') &&
             !contains(github.event.comment.body, '@codebot summarize') &&
             !contains(github.event.comment.body, '@codebot changelog') &&
+            !contains(github.event.comment.body, '@codebot explain-diff') &&
             !contains(github.event.comment.body, '@codebot review-again') &&
             !contains(github.event.comment.body, '@codebot setup-labels')
         runs-on: ubuntu-latest

.gitea/workflows/ai-comment-reply.yml

@@ -1,7 +1,7 @@
 name: AI Comment Reply
 
 # WORKFLOW ROUTING:
-# This workflow handles SPECIFIC commands: help, explain, suggest, security, summarize, changelog, review-again, setup-labels
+# This workflow handles SPECIFIC commands: help, explain, suggest, security, summarize, changelog, explain-diff, review-again, setup-labels
 # Other workflows: ai-issue-triage.yml (@codebot triage), ai-chat.yml (free-form questions)
 
 on:
@@ -24,6 +24,7 @@ jobs:
              contains(github.event.comment.body, '@codebot security') ||
              contains(github.event.comment.body, '@codebot summarize') ||
              contains(github.event.comment.body, '@codebot changelog') ||
+             contains(github.event.comment.body, '@codebot explain-diff') ||
              contains(github.event.comment.body, '@codebot review-again') ||
              contains(github.event.comment.body, '@codebot setup-labels'))
         steps: