[Feature]: Replace pre-commit with lefthook + pixi tasks

### User Story

As a developer,
I want linting and formatting tools managed in a single place (pixi.lock),
So that I don't have duplicate version tracking and I benefit from Renovate's cooldown for supply-chain protection.

### Feature Description

Replace pre-commit (and pre-commit.ci autoupdates) with [lefthook](https://github.com/evilmartians/lefthook), running checks via pixi tasks.

**Motivation:**

- **Duplicate dependency management:** Tool versions (ruff, mypy, codespell, etc.) are tracked in both `.pre-commit-config.yaml` and the pixi environment. With lefthook, versions are pinned once in `pixi.lock`.
- **Supply-chain risk:** pre-commit.ci autoupdates have no cooldown — new hook versions are proposed immediately on release. Moving to pixi-managed dependencies means updates go through Renovate with our existing 7-day `minimumReleaseAge`.
- **Stability:** pre-commit doesn't use lockfiles — it pins the hook repo rev but not transitive dependencies. A hook's dependencies can change or break between runs without any version change in `.pre-commit-config.yaml`. pixi.lock pins the full dependency tree.
- **Simpler CI:** CI already uses pixi. Instead of installing pre-commit separately, CI would just run `pixi run lint` / `pixi run typecheck`.
- **Performance:** lefthook runs hooks in parallel by default and doesn't maintain its own virtualenvs.

### Definition of Done

- [ ] lefthook added as a pixi dependency (`conda-forge::lefthook`)
- [ ] Pixi tasks added for each check (e.g. `ruff-check`, `ruff-format`, `mypy`, `codespell`, `mdformat`)
- [ ] `lefthook.yml` created using the `jobs` API to run formatters first, then linters in parallel
- [ ] `.pre-commit-config.yaml` and pre-commit.ci integration removed
- [ ] CI workflows updated to use pixi tasks directly
- [ ] `conventional-pre-commit` moved to a pip dependency in `pixi.toml`
- [ ] Local hooks (lollygag check, pixi docs generation, settings doc check) ported as pixi tasks

### Alternatives Considered

- **Renovate's pre-commit manager:** Disabled by default indefinitely due to disagreements with the pre-commit project. Would add cooldowns to pre-commit.ci updates but doesn't solve the duplicate dependency management or lockfile issues.
- **Keeping pre-commit with `ci: autoupdate_schedule: never`:** Removes the cooldown gap but still leaves duplicate version tracking and no lockfile for transitive dependencies.

### Additional Context

- lefthook's `jobs` API (v1.10.0+) allows sequential groups with internal parallelism, so formatters run before linters without conflicts
- `conventional-pre-commit` is a pure Python package that can be pip-installed within the pixi environment and called directly from lefthook's `commit-msg` hook