fix: keep IRA prepare lightweight

[EmryZhang]

Summary

• remove the target project's go test ./... execution from IRA prepare
• make prepare archive and inspect the candidate without executing target-project code
• add a 10-minute lightweight CI job for IRA's own unit tests, vet, Go formatting, Bash wrapper syntax, and repository contracts
• update the plan, Skill, documentation, eval, and regression contract to make the boundary explicit

Why

The selected Casbin commit is already tested by its GitHub CI. Repeating the full project test suite inside IRA duplicated that coverage and a recent IRA-hosted run waited for more than 10 hours without package output. IRA should focus on release-specific archive, RAT, checksum, signing, staging, and public verification checks.

Impact

Release preparation no longer creates go-test.log or runs candidate code. Release managers must separately confirm GitHub CI for the exact selected commit. IRA's own code remains covered by the new lightweight CI workflow.

Validation

• go test ./... for the IRA engine
• go vet ./... for the IRA engine
• scripts/validate-repository.ps1
• bash -n skills/incubator-release-assistant/scripts/run.sh ira.sh
• staged diff secret-pattern scan and git diff --check