Final step of the MCreator purge. The MODID (custom_ore_gen) and all
registry names are unchanged, so existing worlds/saves are unaffected:
data JSONs reference the MODID, never the Java package. Updated the
mixin config package and the OresCommandTest package assertion.
- Remove 'MCreator note: REGENERATED' headers and user-code-block
markers (no regeneration step exists anymore)
- Remove MCreator mentions from mod credits/authors
- AGENTS.md: replace the MCreator code-preservation rules with a code
ownership section