fix(release): make worthiness gate pipefail-safe

[levi770]

Summary

• make both release-worthiness history scans consume their full input under set -o pipefail
• keep the existing release subject grammar, with a fail-closed self-check that accepts type(scope)!: and rejects malformed type!(scope):
• leave PR chore: release v0.12.0 #197 and CHANGELOG.md untouched so release-plz remains the changelog owner

Root cause

The review on #197 reproduced git log | grep -q returning pipeline status 141 after grep exits on the first match and git log receives SIGPIPE. The workflow interpreted that matched history as a no-match and stopped refreshing the rolling release PR.

Review reference: #197 (review)

Verification

Before the change, on xlog-cli-v0.11.0..HEAD:

• release subject scan: status 141
• BREAKING CHANGE: scan: status 141

After the change:

• release subject scan: status 0
• BREAKING CHANGE: scan: status 0
• chore-only no-match fixture: status 1
• feat(runtime)!: fixture: status 0
• malformed feat!(runtime): fixture: status 1
• actionlint 1.7.7: clean
• git diff --check: clean

Related to #197; this PR does not close or modify it.

[levi770]

The refreshed release-changelog child is now #265, based directly on this PR's current head cef6aebb3d87437b478e0f789d0b2e5c5d223a3b. Its independently replayed generator converges twice byte-for-byte, and its current-head CI is 10/10 green after adding the PyYAML dependency required by the newly gated docs contract. #265 remains unmerged and does not tag, publish, or close the stale rolling release PR.

[levi770]

Closing this PR because it was opened outside the user-authorized seven-PR scope (#245 and #253-#258). The branch is intentionally retained. If any change here is genuinely required to resolve a review finding, it will be independently verified and folded directly into the relevant original PR rather than published as another PR.