docs: luacheck is a local check now, not a CI one

.github/workflows/lint.yml is gone, so the pointer to it was dangling and
the claim that CI reports luacheck findings on changed lines was no longer
true. Contributors have to run it themselves now, so say that.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SjrTKfA9eJ4S6GJYVKW9mX

Author: keithharvey

.github/copilot-instructions.md

@@ -109,7 +109,7 @@ Match the validator to the file type. If unsure of a tool's scope, inspect the p
 - lux 0.28.x appends duplicate `dependencies` and `entrypoints` entries to `lux.lock` on every cold sync (a run with
   no `.lux/` tree), growing the file by ~13 lines each time without ever converging. Tests still pass. Do not commit
   that churn: `git checkout -- lux.lock` afterwards, and only commit a lockfile change you made deliberately.
-- Lint: `luacheck` 1.2.0 with `.luacheckrc`; CI reports only lines changed in the PR (`.github/workflows/lint.yml`).
+- Lint: `luacheck` 1.2.0 with `.luacheckrc`. Local only — there is no CI job for it, so run it yourself before you push.
 - Format: StyLua with `.stylua.toml` (tabs, indent width 4, 120 columns, CRLF, sorted requires) and `.styluaignore`;
   `.editorconfig` mirrors the indent and whitespace rules.
 - Types: EmmyLua analyzer via `.emmyrc.json`, stubs in `types/`, engine definitions from the `recoil-lua-library`