feat: add /check-golf bot verifying statements are unchanged

[Vilin97]

What

Adds a /check-golf bot that verifies a PR only golfs proofs: that no declaration statement (its signature/type) changed and only proofs and definition bodies changed. Comment /check-golf on any PR and the bot posts its findings as a single comment, editing that same comment on subsequent runs.

Files

• scripts/check_golf.py — compares two revisions of the repo. For every named declaration in the changed .lean files it decides whether the statement was preserved and only the proof/body changed. It also renders the Markdown report and upserts it as a PR comment (POST first, PATCH the marked comment afterwards).
• .github/workflows/check-golf.yml — triggers on an issue_comment containing /check-golf on a PR, resolves the merge-base and head, runs the script, and comments.
• scripts/README.md — documents the tool and how to run it locally.

How it works

The Lean sources are parsed textually (no build required, so the check is fast):

• comments and string literals are blanked, whitespace is ignored;
• top-level declarations are segmented on column-0 command starts (robust even when a proof body contains brackets that cannot be balanced textually, e.g. a by block closed by indentation);
• each statement is split from its body at the top-level := / where / equation | arm, so := in default binders and by blocks inside a type (⟨x, by omega⟩) do not confuse the split;
• by proof terms embedded inside a type are treated as proof-irrelevant, so golfing them counts as a proof change, not a statement change.

Anonymous instances and examples are not tracked (they have no stable name).

Testing

Run locally against two revisions:

scripts/check_golf.py --base <merge-base> --head <head-sha>

Validated against a large real golf PR: the checker reports 0 statement changes across 312 changed files, and a name-level cross-check confirms no named declaration is dropped by the parser. The post→edit comment logic is covered by a mocked-API test.

[github-actions]

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