ci: switch Pre-commit Checks workflow from pre-commit to prek

[potiuk]

Summary

AGENTS.md (since the prek migration) tells contributors to use prek locally and states "the hooks are the same ones CI enforces", but the Pre-commit Checks workflow was still installing pre-commit via uv tool install pre-commit. Swap both lines so CI matches the local dev path: install prek via uv, then prek run --all-files.

Same hooks (prek reads .pre-commit-config.yaml verbatim), same install transport (uv tool install …), just the faster Rust-native runner end-to-end.

Diff

-          uv tool install pre-commit
+          uv tool install prek
           uv tool ensurepath
-          pre-commit run --all-files --color always --show-diff-on-failure
+          prek run --all-files --color always --show-diff-on-failure

Test plan

• CI's pre-commit job (renamed contents, same job name for branch-protection compatibility) runs prek and matches what prek run --all-files produces locally.