Slice 10: create_pr (open a pull request)

forge CreatePullRequest (Gitea; empty base resolves to the repo default branch). Service CreatePR records a pr-created event. HTTP POST /api/repo/pr/create; MCP tool create_pr; <pr-list> New pull request button (head = selected repo current branch, base = default). Live-tested end-to-end: app create_pr opened a real PR (base auto-resolved), listed, then cleaned up; main untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/templates/help.html

@@ -88,7 +88,9 @@
     <h2>Pull requests: Merge &amp; clean up</h2>
     <p>
       When a repository is hosted on your Gitea server, its open pull requests
-      appear under the details panel. Each has a <strong>Merge &amp; clean up</strong>
+      appear under the details panel. Use <strong>New pull request…</strong> to
+      open one from the selected repo's current branch (push the branch first).
+      Each open PR has a <strong>Merge &amp; clean up</strong>
       button: it squash-merges the pull request and <strong>deletes its
       branch</strong> in one step, so finished work doesn't leave branches lying
       around. You'll be asked to confirm — it names the pull request and the branch