Add `CONTRIBUTING.md`

[DevanshSrajput]

Context
There's no CONTRIBUTING.md at the repo root. New contributors
have to read the README, the test docstring, and pyproject.toml
to figure out: how to run the tests, where to log an issue,
the PR conventions (commit message style, branch naming, "no
ISSUES.md in commits"), and how to add a new model id safely.
The README covers most of it implicitly but not for contributors.

What to change
Create CONTRIBUTING.md at the repo root with four short
sections:

1. Setup — clone, venv, pip install -r requirements.txt,
   python -m unittest discover -s tests -v.
2. Tests — the dual-runner docstring note (pytest + unittest),
   the STREAMLIT_RUN=1 env var, the numpy/pandas skip behaviour.
3. PR conventions — one commit per fix, brief messages,
   ISSUES.md is personal tracking and must NOT be committed.
4. Adding a new model — the curl recipe from
   app_helpers.AVAILABLE_MODELS docstring, plus the test
   to update.

How to verify

• head -5 CONTRIBUTING.md describes setup in one paragraph.
• The doc references the actual commands, not paraphrases.
• A new contributor can clone, install, run tests, and open a
  PR using only this file (no Slack, no asking around).

Skill: docs.
Estimated effort: S.