chore(release): add release preparation flow

[samzong]

Summary

Add a maintainer release preparation flow for kitup:

• Switch the root Makefile to a default help view with release prep targets.
• Add scripts/prepare-release.mjs for branch creation, version bumping, checks, and signed release-prep commits.
• Document the manual PR and post-merge tag flow in docs/RELEASE.md, CONTRIBUTING.md, and .agents/skills/kitup-release/SKILL.md.
• Extend release automation and smoke checks to cover the Go Cobra module.

Scope Check

• Stays within the v0.1 bundled-skill installer boundary.
• Does not add marketplace, registry, remote install, script execution, GUI, MCP server, or agent runtime behavior.
• No observable installer behavior changed, so no golden cases were added.
• No spec/hosts.json changes, so host constants were not regenerated.

Verification

• make check
• make help
• node scripts/prepare-release.mjs patch --dry-run
• node scripts/prepare-release.mjs minor --dry-run
• node scripts/prepare-release.mjs major --dry-run
• sh -n scripts/smoke-release.sh
• git diff HEAD~1..HEAD --check
• Pre-ship committed diff scan: 0 MUST-FIX findings