lint: what do we do with the checks/ directory once Ruff is required?

[Huoyanlifusu]

Problem

checks/ currently reports ~1374 violations under Ruff, but 1162 of them are F821 (undefined-name) — which is just how the checks work: they reference names the learner hasn't defined yet, and the runner's shared-namespace mechanism resolves them at runtime. So that count is mostly noise.

The rest is roughly 212 real-ish ones:

• 159 × E501 (long lines)
• 18 × E712 (comparing with == True etc.)
• 13 × E402 (imports not at top)
• 11 × I001 (unsorted imports)
• ~11 others

But these files are part of the curriculum: they're mirrored with exercises/ and solutions/ and get copied into learner workspaces. I don't want to reformat them blindly and end up with noisy diffs or confusing changes for learners.

Proposed Behavior

Before doing anything, I'd like to agree on the policy. My suggestion:

1. Per-file-ignore F821 for checks/** (it's expected, not a bug):

[tool.ruff.lint.per-file-ignores]
"checks/**/*.py" = ["F821"]

2. Leave the remaining ~212 alone for now and decide later whether they're worth fixing (probably yes, but as its own pass, keeping the exercise/check/solution mirrors in sync).

Alternatives Considered

• Fixing everything in checks/ — not possible for F821, and risky for the rest.
• Excluding checks/ from Ruff entirely — simplest, but we'd lose visibility into genuinely fixable stuff.
• Doing nothing — fine while lint is advisory, but we need a decision before ci: add automated style linting (ruff) so style issues are caught before review #124 makes it required.

[abhiksark]

I am planning to do this during version release.