chore: release 0.5.1

[astykyj]

Summary

• Repair the v0.5.0 baseline tag onto the current main history so incremental changelog generation uses the intended release range.
• Bump Stave to 0.5.1.
• Add generated changelog notes for the branch selector, GPT-5.5 default, release repo, and Node-compatible script guard changes.

Verification

• bun run typecheck
• bun test tests/top-bar-branch-dropdown-utils.test.ts tests/model-selector-utils.test.ts tests/model-shortcuts.test.ts tests/task-presets.test.ts tests/install-guide.test.ts tests/scripts-smoke.test.ts

[heath-s]

Approved