[crs7617]

Fixes #290

Problem
link --create-pr printed "PR creation not yet implemented" and then called
upload_file without create_pr, so the README change landed on main.

Change

• Pass create_pr through to HfApi.upload_file and keep the returned CommitInfo.
• In PR mode, print "Opened PR" with the PR URL and add pr_url to the JSON output
  (main() prints that dict). Direct-commit output is unchanged.

Testing

• Added scripts/test_paper_manager.py (unittest, mocked HfApi, no network or token).
• (a) create_pr=True forwards the flag and reports the PR URL. It fails without the
  fix and passes with it.
• (b) The default path still commits directly. This is a regression guard and passes
  before and after.
• Existing test_build_skill_distribution.py still passes.
• Not tried against the real Hub.

Not addressed

• With several arXiv IDs plus --create-pr, each paper opens its own PR against main,
  so they may conflict once one is merged.
• If the paper is already in the README, the script still uploads an unchanged file.
  That already happens in direct mode.
• The README is always read from main, not from an open PR branch.
• The new test isn't run by CI, which only runs test_build_skill_distribution.py.
  Happy to wire it in if you want.

Thanks @JacobTRND for the report and diagnosis. I used Claude Code to help
investigate and draft; I reviewed the diff and ran the tests myself.