ci: fail loudly on malformed landed release subjects

[rianjs]

Summary

• distinguish valid non-release conventional subjects from malformed landed subjects in the shared release-gate checker
• preserve feat/fix release eligibility and docs/refactor/ci skip behavior
• add regression coverage for release, skip, and fail-loud cases

Root cause

The auto-release action treated every release-gate exit 1 as an intentional skip. A non-conventional landed subject could therefore green-skip auto-release even when the PR title was conventional. Malformed subjects now return an error status with a targeted message, which the existing action wrapper fails loudly.

Checks

• bash actions/auto-release/test_gate.sh
• bash -n actions/conventional-commit/check.sh
• bash -n actions/auto-release/test_gate.sh
• actionlint
• git diff --check

The repository Python test command could not run locally because the environment has no installed pytest module; the workflow installs its pinned test dependencies.

Closes #36