[codex] Launch external validation recruitment

[echowang1]

Summary

• Add the approved external validation recruitment design and execution plan.
• Add an MIT license and contribution guidance for public validation.
• Add actionable scenario and integration feedback links.
• Track public-readiness, candidate sourcing, and first-wave outreach tasks.

Validation

• Public-exposure audit of tracked files and all reachable Git history
• GitHub-side audit of PRs, Actions logs, artifacts, releases, comments, variables, and secrets metadata
• npm run typecheck
• npm test
• npm run build
• git diff --check

Scope

This PR prepares the repository for public external validation. Candidate contact data and outreach records remain local and excluded from Git.