#3355 docs: record that compile-check stays its own command and is shown

Summary

• Records the decision that uloop compile-check stays a command of its own (not an option of uloop compile) and becomes visible in help, uloop list, completion and skills.

User Impact

• No behavior change in this PR. It fixes the scope the following PRs implement before the command is shown, so a reader can tell what a refusal means and which changes still need uloop compile.

Changes

• Adds ADR 0018: why compile-check is not a compile flag, the condition for showing it, what it detects as COMPILE_CHECK_UNITY_BUILD_REQUIRED, what it compiles rather than narrows, the limitations that stay documented, and when to revisit.
• The follow-up PR-1 to PR-6 on this integration branch implement this ADR's detection scope (lost asmdef references, build settings recorded against the last Unity build, records of skipped dependents, unicode escapes in identifiers, new scripts outside every assembly definition) and then show the command.

Verification

• ls docs/adr/0018-* lists one file.
• grep -n 'compile --check\|compile --offline' docs/adr/0018-* finds nothing.