ci: make downstream version-bump branches semver-policy compatible #2484

Summary

Make Base downstream version-bump automation generate branch names accepted by the downstream issue-branch policy.

Problem

The generator currently creates branches such as ci/348-20261008-bump-base-bash-libs-2.2.2. base-demo requires lowercase alphanumeric hyphen-separated slugs and rejects the dot characters, leaving PRs #345, #347, and #349 blocked with branch name is not canonical.

The same defect will block the first generated base-cli 0.5.0 downstream bump.

Acceptance criteria

• Generated branch slugs are canonical for semver component versions.
• The generated branch remains issue-backed and deterministic.
• Workflow tests cover patch/minor/major versions and the downstream policy regex.
• The v2.2.2 downstream bump can be regenerated and validated.

Release impact

This blocks automated alignment of base-demo before the Base 1.10.0 release.