perf(languages): reuse baked Go and Java

[rldyourmnd]

Summary

• add opt-in exact preinstalled Go/gofmt verification
• add opt-in baked Java/Maven verification
• skip setup actions only on explicitly selected immutable ephemeral runners
• preserve hosted setup and caching defaults

Permissions diff

None.

Threat model

Preinstalled mode is explicit and version-verifying; hosted and mutable runners retain setup actions. Caller commands remain environment-bound and fail-fast.

Validation

• actionlint
• all validation tiers with live ruleset access
• zizmor 1.26.1 pedantic/low — no findings
• git diff --check