Remove the conda-based installation scripts and documentation now that
`pip install singlebehaviorlab` covers the full stack.

- Delete install.sh, run.sh, requirements.txt, environment.yml,
  INSTALL.md, and the main.py compatibility launcher. Their jobs are
  all covered by `pip install` and the singlebehaviorlab console entry
  point.
- Rewrite the README installation and launch sections around pip,
  with a short "Development install" subsection for contributors.
- Update the README directory-structure diagram to reflect the
  pip-install layout (third_party/ for vendored code, licenses/ for
  Apache attribution).
- Drop the INSTALL.md link from HOWTOUSE.md and the stale
  `bash run.sh` / `python main.py` launch options.
- Clean up .gitignore: replace sam2_backend/ patterns with the
  third_party/ paths.