Register entry point and verify integration

[abwaters]

Description

Add the plugin entry point to pyproject.toml, reinstall the package, and verify the full test suite passes.

Acceptance Criteria

• Entry point added: github-repo-filter = "mcp_zero.plugins.github_repo_filter:GitHubRepoFilterPlugin" under [project.entry-points."mcp_zero.plugins"]
• Package reinstalled with pip install -e ".[dev]"
• ruff format src tests passes
• ruff check src tests passes
• All new tests pass: python -m pytest tests/plugins/test_github_repo_filter.py -v
• Full test suite passes with no regressions

Parent story: #141

[abwaters]

All acceptance criteria verified:

• Entry point already registered in pyproject.toml (line 27)
• Package reinstalled with pip install -e ".[dev]"
• ruff format src tests — all 148 files clean
• ruff check src tests — all checks passed
• Full test suite: 1105 passed in 169.75s, zero regressions