fix: remove customize opencode core skill

[Alezander9]

Fixes the release build failure caused by core importing the deleted built-in customize-opencode skill markdown.\n\nChanges:\n- Stop booting the core skill plugin.\n- Make the stale core skill plugin module a no-op if imported directly.\n- Remove the test that asserted registration of the upstream built-in customize-opencode skill.\n\nVerification:\n- bun run typecheck\n- bun run packages/opencode/script/build.ts --single --skip-install --skip-embed-web-ui

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Summary by cubic

Fix release build failure by removing the built-in customize-opencode core skill. The core no longer boots the skill plugin, and the plugin is now a no-op to avoid importing the deleted markdown.

• Bug Fixes
  • Removed SkillPlugin.Plugin from the core boot sequence.
  • Set SkillPlugin.Plugin effect to Effect.void to make direct imports safe.
  • Deleted test that asserted registration of the "customize-opencode" built-in skill.

^{Written for commit b832be8. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}