Adopt Release Please for automated software-agent-sdk releases

Problem

The SDK release process still relies on manually dispatching prepare-release.yml with an explicit version, maintaining a rel-X.Y.Z branch, and then merging that release PR to create and publish the release. This adds coordination overhead and differs from the Release Please workflow used by other OpenHands repositories.

Proposed solution

Adopt the shared OpenHands/release-actions Release Please workflow for software-agent-sdk.

The implementation should:

• maintain a draft Release Please PR from pushes to main and supported release/** branches;
• derive semantic versions from conventional PR titles;
• keep openhands-sdk, openhands-tools, openhands-workspace, and openhands-agent-server versions in lockstep;
• use the release PR's ready-for-review transition to apply the integration, behavior, and example-test labels and notify the SDK release channel;
• create the GitHub tag and release on merge, continuing to trigger PyPI publication, agent-server images, release binaries, and downstream version bumps;
• update the release documentation and remove the superseded manual preparation/creation workflows.

Acceptance criteria

• Release Please creates and maintains a draft release PR on main.
• A dry run verifies all four package pyproject.toml versions and the manifest/version file are updated together.
• Marking the release PR ready triggers the intended release test labels without continuously rerunning expensive tests.
• Merging the release PR creates a valid vX.Y.Z tag and GitHub release.
• PyPI, images, binaries, and downstream bump workflows still run successfully.
• Release documentation describes the new process.

Existing implementation

PR: https://github.com/OpenHands/software-agent-sdk/pull/3939