docs: strengthen OSS positioning and discoverability

[hcsolakoglu]

Summary

• Rewrite the README opening around the project's actual OSS value: Codex usage visibility, OpenAI-issued reset credits, multi-account workflows, automation, and redemption safety.
• Make the trust boundary explicit: this tool does not create credits, bypass plan limits, or unlock unsupported access.
• Surface upstream-contract tracking, weekly drift detection, cross-platform CI, and npm OIDC provenance near the top instead of burying them.
• Improve npm package description and search keywords without inventing usage or adoption claims.
• Preserve installation, command, auth, security, compatibility, and contribution documentation while making the README easier to scan.

Why

The existing documentation was technically detailed but did not quickly communicate the project's ecosystem role, maintenance practices, and supply-chain posture. This change improves GitHub/npm discoverability and makes the repository easier for users and OSS reviewers to evaluate.

Integrity

No stars, download counts, contributor counts, endorsements, or other adoption metrics are fabricated or hard-coded. The project is explicitly identified as independent and unofficial.

Validation

Documentation/package-metadata-only change. CI should run the existing typecheck, lint, tests, build, package dry-run, and cross-platform matrix.