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

[chrisburr]

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, 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