ci: allow PyPI deps during CPU torch install (#1930)

## Description

Fixes the CI failure exposed on the `8527b910` push run:

- Run:
https://github.com/headroomlabs-ai/headroom/actions/runs/29063619700
- Failed job: `test (3)` / job `86271743483`
- Failed step: `Install (CPU torch + prebuilt wheel + dev deps, no cargo
rebuild)`

The failing command used the PyTorch CPU index as the only package
index:

```bash
pip install torch --index-url https://download.pytorch.org/whl/cpu
```

That index does not provide transitive dependencies such as
`typing-extensions`, so pip backtracked across torch CPU wheels and
failed before tests ran. This PR keeps the PyTorch CPU index primary for
`torch` while allowing PyPI for dependencies.

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Performance improvement
- [ ] Code refactoring (no functional changes)

## Changes Made

- Adds `--extra-index-url https://pypi.org/simple` to each CI `pip
install torch --index-url https://download.pytorch.org/whl/cpu` command.
- Leaves the CPU torch index as the primary index so CI still installs
CPU torch wheels.

## Testing

- [ ] Unit tests pass (`pytest`)
- [ ] Linting passes (`ruff check .`)
- [ ] Type checking passes (`mypy headroom`)
- [ ] New tests added for new functionality
- [x] Manual testing performed

### Test Output

```text
git diff --check
# passed

rg -n "pip install torch" .github/workflows/ci.yml
# all four torch install commands now include --extra-index-url https://pypi.org/simple
```

## Real Behavior Proof

- Environment: Local worktree `C:\git\headroom-ci-fix`, branch
`jd/fix-ci-torch-install-index`.
- Exact command / steps: Inspected failed GitHub Actions log for run
`29063619700` job `86271743483`, then updated matching torch install
commands in `.github/workflows/ci.yml`.
- Observed result: The workflow no longer uses the PyTorch CPU index as
the only index for torch installs; PyPI is available for transitive
dependencies such as `typing-extensions`.
- Not tested: Full CI run locally; GitHub Actions is the authoritative
verification for the affected install path.

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] I have updated the CHANGELOG.md if applicable

## Screenshots (if applicable)

N/A.

## Additional Notes

This is a fix-forward PR. Reverting `8527b910` would reintroduce the
Ruff failure it fixed; the failed job did not reach tests and failed
only during dependency installation.

.github/workflows/ci.yml

@@ -201,7 +201,7 @@ jobs:
       - name: Install (CPU torch + prebuilt wheel + dev deps, no cargo rebuild)
         run: |
           python -m pip install --upgrade pip
-          pip install torch --index-url https://download.pytorch.org/whl/cpu
+          pip install torch --index-url https://download.pytorch.org/whl/cpu --extra-index-url https://pypi.org/simple
           WHEEL="$(ls dist/*.whl)"
           pip install "${WHEEL}[dev]" pytest-split
           # cwd's ./headroom source tree shadows the installed wheel; copy the
@@ -275,7 +275,7 @@ jobs:
       - name: Install (CPU torch + wheel[dev,relevance])
         run: |
           python -m pip install --upgrade pip
-          pip install torch --index-url https://download.pytorch.org/whl/cpu
+          pip install torch --index-url https://download.pytorch.org/whl/cpu --extra-index-url https://pypi.org/simple
           WHEEL="$(ls dist/*.whl)"
           pip install "${WHEEL}[dev,relevance]"
           SITE="$(python -c 'import sysconfig; print(sysconfig.get_path("platlib"))')"
@@ -317,7 +317,7 @@ jobs:
       - name: Install (CPU torch + wheel[dev,agno])
         run: |
           python -m pip install --upgrade pip
-          pip install torch --index-url https://download.pytorch.org/whl/cpu
+          pip install torch --index-url https://download.pytorch.org/whl/cpu --extra-index-url https://pypi.org/simple
           WHEEL="$(ls dist/*.whl)"
           pip install "${WHEEL}[dev,agno]"
           SITE="$(python -c 'import sysconfig; print(sysconfig.get_path("platlib"))')"
@@ -343,7 +343,7 @@ jobs:
       - name: Install (CPU torch + wheel[dev] + playwright)
         run: |
           python -m pip install --upgrade pip
-          pip install torch --index-url https://download.pytorch.org/whl/cpu
+          pip install torch --index-url https://download.pytorch.org/whl/cpu --extra-index-url https://pypi.org/simple
           WHEEL="$(ls dist/*.whl)"
           pip install "${WHEEL}[dev]" playwright
           SITE="$(python -c 'import sysconfig; print(sysconfig.get_path("platlib"))')"