feat: add publish action outputs

[damacus]

Summary

• declare composite action outputs for releases, version, and completed
• write no-secret machine-readable output values from script/publish
• document the output contract and cover success/failure output behavior in tests

Verification

• bash test/publish_test.sh
• bash script/check
• git diff --check

Stacked on #3 because this depends on the hardened script/publish implementation.