Prepare gitbriefcase release candidate surface

[rogerchappel]

Summary

• include support and release notes in the npm package allowlist
• make package smoke rebuild and assert the published CLI/API entrypoints before dry-run packing
• align README release readiness guidance with the stronger package smoke command

Verification

• npm ci
• npm run release:check
• package smoke confirmed dist/src/cli.js and dist/src/index.js and listed SUPPORT.md/RELEASE_NOTES.md in npm pack --dry-run

Notes

Suggested GitHub topics: git, support-bundle, agent-tools, redaction, cli, local-first