Four behavior fixes from main's inline-renderer architecture, re-expressed
in the branch's extracted ChatMarkdown factory where the rebase could not
carry them:

- #7741: file-link tooltips show the full target path, not the shortened
  display form the chip already shows
- #7856: ordered-list marker gutters size by marker width (string starts,
  negative markers, first-vs-last width) instead of digit count alone
- #6439: link-href extraction uses the shared spaced-path-aware helper,
  so command-clicking 'my folder/file.ts' links works
- #8098: file links only default to the in-app browser for browser-native
  documents; agent file links open in the file viewer again

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>