Show wiki diff in PRs created by agentic-wiki-coder

[mrjf]

Summary

• Adds wiki commit diff URLs ({html_url}/_compare/{sha}) to PRs created by the wiki-to-code workflow
• New Step 1b fetches the actual diff content from the wiki git clone
• PR body template now leads with the wiki diff — shown inline if small, summarized + collapsed if large
• Same format applies if the safe-output falls back to an issue

Test plan

• Trigger agentic-wiki-coder with a small wiki edit and verify the PR body shows the diff inline with a link to the _compare view
• Trigger with a large wiki edit and verify the diff is summarized above the fold with full diff in a <details> block

🤖 Generated with Claude Code