chore: add ruff pre-commit, commitizen, and CI lint gate

[tomtranjr]

summary

Adds developer tooling on top of the uv migration (#67): a local pre-commit hook that auto-fixes lint/format, a commit-msg hook that enforces Conventional Commits (which python-semantic-release now depends on), and a CI lint gate that only checks (never fixes).

• .pre-commit-config.yaml: ruff v0.15.21 (ruff-check --fix + ruff-format) at the pre-commit stage, commitizen at the commit-msg stage. default_install_hook_types wires both up with one pre-commit install.
• pyproject.toml: ruff==0.15.21 (pinned to match the hook) + pre-commit added to the dev group; minimal [tool.ruff] using defaults, recycling_bin/ excluded as dead code.
• .github/workflows/release.yml: new lint job (ruff check . + ruff format --check ., no --fix); release now needs [lint, test].
• docs/contributing.md: pre-commit setup step + note that CI enforces the same checks.
• One-time cleanup so the gate is green: style: commits apply ruff format + autofixes; fix: commit resolves the 15 non-auto-fixable lint errors (unused vars, bare excepts, ambiguous name, mid-file imports, == True).

why draft / base branch

Based on #67's branch, not main, since #67 rewrites pyproject.toml and the CI workflow this touches. Draft for teammate visibility while #67 is in review. Retarget base to main after #67 merges.

division of labor

	local pre-commit	CI gate
ruff lint	ruff check --fix (fixes)	ruff check (fails, no fix)
ruff format	ruff-format (rewrites)	ruff format --check (fails)
commit msg	commitizen (commit-msg)	not in CI

test plan

• uv run ruff check . and uv run ruff format --check . clean
• uv run pytest — 65 passed
• uv run pre-commit run --all-files passes
• commit-msg hook rejects non-conventional messages, accepts conventional