[CI]: Add CI workflow with lint, type check, and test matrix

[spencrr]

Description

Add GitHub Actions CI workflow with lint, type checking, and test matrix across
Python 3.11–3.13.

Lint & Type Check job:

• Runs on ubuntu-latest with Python 3.12
• Uses uv sync --frozen for reproducible installs
• Runs ruff check, ruff format --check, and pyright in sequence

Test matrix job:

• Runs on Python 3.11, 3.12, and 3.13 with fail-fast: false
• Per-version uv cache (cache-suffix: py${{ matrix.python-version }})
• Runs pytest tests/unit with coverage collection (--cov=rampart)
• Coverage threshold enforced via [tool.coverage.report] fail_under in pyproject.toml

Workflow configuration:

• Triggers on push to main and PRs targeting main
• concurrency group with cancel-in-progress: true to avoid redundant runs
• Empty default permissions: {} with contents: read scoped per-job
• All actions pinned by SHA with version comments

Breaking changes

None

Checklist

• pre-commit run --all-files passes
• Tests added or updated for changes
• Documentation updated