fix(release): generate signed Release Please proposal commits #44

Release Please cannot update its release proposal branch under the repository's signed-commit requirement. This predates the API v0.5.0 SDK adoption in #41 and blocks the next normal package release.

Evidence:

• Failed release workflow fails while updating release-please--branches--main--components--morphiq-sdk.
• Its generated commit 748e987cffcd926fe3117896da05716cc8dc9aa1 is reported by GitHub as verified: false, reason unsigned.
• The effective release-branch rules include required_signatures (verified through the GitHub branch-rules API on 2026-11-04).
• The current action uses the shared RELEASE_PLZ_TOKEN; local signed feature commits are accepted and verified normally.

Restore automated release proposals with verified signatures, while preserving Release Please ownership of version/manifest/changelog changes and the normal PR/CI/release flow. Keep the signed-commit policy and release-tag protections in place; do not solve this by manually bumping versions, cutting tags or bypassing checks.

Acceptance: a normal main-branch release workflow produces a verified release-PR commit, its Python/Docker checks pass, and the reviewed release can proceed through the existing TestPyPI/PyPI pipeline. No package was published as part of #41.