ci: run pre-commit on pull requests

[mscolnick]

What / why

This repo ships a .pre-commit-config.yaml whose hooks were not consistently enforced. This PR makes the pre-commit hooks run reliably in CI and green.

Changes

1. ci: skip: [go-fmt, go-vet] in .pre-commit-config.yaml

This is the key fix. Go formatting/vetting is already enforced by the golangci-lint workflow (lint.yml), and the dnephin/pre-commit-golang go-vet hook is broken for this repo's layout — it fails with no Go files in <root> because the packages live in subdirectories under a single root go.mod. Skipping both Go hooks on pre-commit.ci makes that status green; Go quality stays covered by golangci-lint.

2. A native pre-commit GitHub Actions workflow

Runs uvx pre-commit run --all-files --show-diff-on-failure via astral-sh/setup-uv on PRs and pushes to main (pre-commit.ci only covers PRs). Read-only, SHA-pinned, caches ~/.cache/pre-commit. It uses SKIP: go-fmt,go-vet for the same reason as above. (If you'd rather rely solely on pre-commit.ci, this workflow can be dropped — the ci: skip change above is the essential part.)

3. check-yaml multi-document support

Added args: [--allow-multiple-documents] — the repo's k8s manifests (config/manager/manager.yaml, deploy/install.yaml, examples/git-sync-sidecar/notebook.yaml) are valid multi-document YAML that the default hook rejected.

4. Mechanical hook autofixes (previously unenforced)

• ruff (auto): removed unused imports (F401) in plugin/examples/with-cw.py, plugin/tests/test_status.py; removed extraneous f-string prefixes (F541) in plugin/kubectl_marimo/deploy.py.
• ruff (manual, trivial): dropped an unused mock_echo assignment (F841) in plugin/tests/test_deploy.py (kept the mocker.patch(...) call).
• whitespace/EOF fixers: trailing blank line in .devcontainer/devcontainer.json, missing trailing newline in hack/boilerplate.go.txt.

Verification (local, Go hooks skipped as scoped)

trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
check yaml...............................................................Passed
check for merge conflicts................................................Passed
ruff.....................................................................Passed
ruff-format..............................................................Passed
go fmt..................................................................Skipped
go vet..................................................................Skipped

Part of the marimo-team engineering-excellence initiative to make sure every repo's declared quality gates actually run — and pass — in CI.

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