fix(deps): clear Dependabot lockfile alerts (#2175)

## Description

Clears the current dependency/security-audit blockers that are making
unrelated PRs red:

- `transformers 5.3.0` / `CVE-2026-5241`, fixed by requiring
`transformers>=5.5.0` in the locked optional dependency set.
- `sqlitedict <=2.1.0` via the optional `benchmark` extra's
`lm-eval[api]` dependency. There is no patched `sqlitedict` release, so
this PR removes the published/locked `benchmark` extra instead of
shipping a known-vulnerable transitive dependency.
- `esbuild >=0.27.3,<0.28.1` in the OpenCode plugin lockfile, fixed by
forcing `esbuild@0.28.1` through the OpenCode npm override and
regenerated lockfile.

The benchmark code still invokes `python -m lm_eval`; researchers who
need that harness should install `lm-eval[api]` in their benchmark
environment until its transitive vulnerability has a patched release.

## 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

- `pyproject.toml`: remove the `benchmark` optional extra, document
external `lm-eval[api]` installation guidance, and require
`transformers>=5.5.0`.
- `uv.lock`: regenerate without the `benchmark` extra, removing
`lm-eval` and `sqlitedict` lock entries and locking the patched
transformers floor.
- `plugins/opencode/package.json`: add an `overrides` entry for
`esbuild@0.28.1`.
- `plugins/opencode/package-lock.json`: regenerate the OpenCode lockfile
with `esbuild@0.28.1`.

## Testing

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

### Test Output

```text
uv lock --check
rg -n -F 'sqlitedict' uv.lock        # no matches
rg -n -F 'name = "lm-eval"' uv.lock  # no matches
rg -n -F "extra == 'benchmark'" uv.lock # no matches
rg -n -F '0.27.7' plugins/opencode/package-lock.json plugins/opencode/package.json # no matches
npm ls esbuild --package-lock-only
npm audit --package-lock-only        # found 0 vulnerabilities
git diff --check
```

Previous GitHub checks were green. After merging current `main`, fresh
GitHub checks are running again; local targeted validation still passes.

## Real Behavior Proof

- Environment: Windows 11, Python 3.13.3, uv, npm in `plugins/opencode`,
Dependabot/pip-audit alert metadata from the failing PR jobs.
- Exact command / steps: inspected the regenerated Python and npm
lockfiles with `rg`, checked the uv lock with `uv lock --check`, checked
OpenCode's dependency tree with `npm ls esbuild --package-lock-only`,
and ran `npm audit --package-lock-only`.
- Observed result: `uv.lock` no longer contains `sqlitedict`, `lm-eval`,
or a `benchmark` extra marker; `transformers` resolves at the patched
`>=5.5.0` floor; OpenCode's lock resolves `esbuild@0.28.1`; `npm audit
--package-lock-only` reports 0 vulnerabilities; GitHub `Dependency audit
(pip-audit)` passes.
- Not tested: running the external `lm-eval` harness after installing it
separately.

## 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
- [x] 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
- [x] New and existing unit tests pass locally with my changes
- [x] I have updated the CHANGELOG.md if applicable

## Screenshots (if applicable)

N/A - dependency and lockfile security fix.

## Additional Notes

The `benchmark` extra can be restored once the upstream `lm-eval[api]`
dependency chain stops pulling a vulnerable `sqlitedict` release.

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

plugins/opencode/package.json

@@ -42,5 +42,8 @@
     "typescript": "^5.5.0",
     "vitest": "^4.1.10"
   },
+  "overrides": {
+    "esbuild": "^0.28.1"
+  },
   "license": "Apache-2.0"
 }