chore: rename LICENCE to LICENSE so GitHub detects BUSL-1.1

GitHub's licence detection only matches the American spelling, so the
repository currently reports "Other" / NOASSERTION despite carrying a
standard Business Source License 1.1. Renaming the file makes the licence
show in the repo sidebar, the API, and downstream tooling that reads it.

No change to the licence text or terms.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: yieldhunting