Prepare Marketplace 0.2.5

[veil-chow-fyaic]

Summary

• bump the Marketplace Preview package to 0.2.5
• remove stale 0.2.3 copy from the packaged README and feature contract
• update the manual Marketplace gate and issue template
• clarify four-platform package selection and record release-candidate evidence

No extension runtime behavior changes. The bundled helper remains Agent Kit 0.4.4.

Verification

• clean npm ci and high-severity audit: 0 vulnerabilities
• source verifier: 56 files, Agent Kit 0.4.4
• Marketplace verifier: 4 tests
• TypeScript/build and 18 contract tests
• VS Code Extension Host smoke: exit 0
• darwin-arm64 VSIX verification and production HTTPS health check