[P1] Add release checklists for client, backend, SDK, and contracts

[Calebux]

Summary

Each package has different operational risks and release steps.

Scope

• Area: docs
• Priority: P1
• Backlog ID: #115

Acceptance Criteria

• Each release surface has a checklist.
• Checklist includes verification, rollback, and comms steps.
• Release docs are linked from the main contributor guide.

Implementation Notes

• Keep the implementation aligned with current architecture decisions and security constraints in this repo.
• Include tests for changed behavior and update docs where relevant.

Definition of Done

• Acceptance criteria met
• Tests added/updated and passing
• Documentation updated
• No security regressions introduced

Links

• Backlog source: docs/repo-issue-backlog-2026-05.md