fix: add lint rule to detect bare pip install and fix beval.yml violation

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### Component

Scripts / Workflows

### Description

A companion to #1313, which narrows scope to instruction file updates. This issue owns the enforcement backstop and the one known existing violation.

**Background:** #1313 adds explicit `pip install` prohibitions to the Python instruction files. Without a validation mechanism, those rules have no CI enforcement and regressions can silently appear.

**Known violation:** `.github/workflows/beval.yml` contains a bare `pip install --no-cache-dir "beval[all] @ git+..."` call that should be replaced with a `uv`-managed equivalent to comply with the repo's uv-first Python convention.

### Acceptance Criteria

- [ ] A new reusable workflow (`.github/workflows/pip-install-lint.yml`) runs a script that detects bare `pip install` calls in `.github/workflows/`, `scripts/`, and `.py` files
- [ ] A corresponding job is added to `pr-validation.yml` to invoke the new reusable workflow on every PR targeting `main`
- [ ] The lint check excludes: `uv pip install`/`uv pip freeze` forms, files under `evals/` that reference `pip install` as failure-signal strings, and `THIRD-PARTY-NOTICES`
- [ ] `.github/workflows/beval.yml` is updated to use the `uv`-managed equivalent of the `beval` install
- [ ] The new lint check passes clean against the updated repo

### Additional Context

`npm run lint:all` is a local developer convenience aggregator and is not called by any CI workflow. New lint checks require both a reusable workflow file and an explicit job entry in `pr-validation.yml` to run on PRs.

Start by reviewing .github/workflows/pr-validation.yml and the existing workflow conventions, then inspect .github/workflows/beval.yml and the scoped workflow, scripts, and Python files for bare pip install calls. The work is done when .github/workflows/pip-install-lint.yml and its PR-validation job enforce the stated exclusions, beval.yml uses the uv-managed form, and the check passes cleanly.

Written by the indexing model from the issue text.