Turn the code tab into an editor-style file browser

Turn the code tab into an editor-style file browser

- Add a persistent nested file tree with HTMX file and folder swaps, active
  rows, expanded ancestors, and deep-link support.
- Share one page header across every repository tab: title and summary on the
  left, tab actions grouped on the right.
- Make the repository name the first tab and show visibility with a lock or
  globe icon derived from the ACL.
- Open markdown as syntax-highlighted source by default, with an explicit
  full-width preview toggle.
- Point the branch breadcrumb at the Code overview instead of a second root
  directory listing.
- Move contributor-branch CI approval from the code browser to the Runs tab.
- Extend UI tests for the file tree, fragments, repo visibility, markdown
  source/preview, shared headers, and the moved CI control.