chore(ci): commit composer.lock for deterministic CI resolution (#15)

Un-gitignore /composer.lock and commit a fresh lock so CI installs the
same dependency graph every run. Previously the lock was gitignored and
CI ran `composer install --prefer-dist`, which with no committed lock
silently degrades to a full fresh resolve on every run — permissive
ranges (illuminate/* ^11||^12||^13, testbench ^9||^10||^11) let a
transitive major land in CI unbidden with no source change.

Published client library: consumers resolve against their own
constraints, so this lock is never consumed downstream (zero consumer
impact). Dependabot composer keeps the committed lock fresh via reviewed
PRs. Sister to phpstan-warroom-rules PR #55.


Claude-Session: https://claude.ai/code/session_0129qYnTu3C5qVK2yNcqHELD

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Goosterhof

.gitignore

@@ -1,7 +1,6 @@
 /vendor/
 /.phpunit.cache/
 /.phpunit.result.cache
-/composer.lock
 /.idea/
 /.vscode/
 /build/