Build out change review and threaded discussions

Build out change review and threaded discussions

- Add Conversation, Files and Runs tabs to change requests. Files uses the
  code-browser shape with a changed-file tree and one selected-file diff;
  Runs lists the head commit's workflow results.
- Show anchored review comments beneath the line, with a collapsed excerpt
  read from the exact blob that was reviewed, and return posted anchors to
  their line note.
- Add comment threads: any comment can be replied to, the UI flattens the
  descendants under one root, reply context links back to the parent, and
  reply targets feed a single composer at the foot of the discussion.
- Collapse reply groups while keeping the root visible, and scroll to the
  composer when Reply is selected.
- Add comment context menus with Reply, Copy text, Copy link, Edit and
  Delete. Add web edit/delete routes with author or admin checks; deletion
  leaves a tombstone so replies keep their thread.
- Support --reply-to in the issue and change CLI commands, and treat a
  missing parent as a tombstone rather than an invalid tree.
- Fix the diff comment gutter, action button colours, reply labels, and
  keep the Files tree and diff pane aligned.

Update metadata validation, web templates, CSS, JavaScript, tests, and the
CLI/change-request/issue docs.