Fix subject-verb agreement in the README introduction

[RobGrafrath]

What this changes and why

The README's opening paragraph pairs a three-part compound subject with a singular verb: "Every decision, its reasoning, and its implications for contributors is public here." The final item, implications, is plural, so the verb should be plural. This change replaces "is" with "are" and touches nothing else. The pull request doubles as the shakedown of the repository's checks before outside contributions arrive.

Interacting decisions

None. This edits README.md prose only, and I checked INDEX.md to confirm no decision file is involved.

Version bumps

None. No decision file changes.

Checklist

• I read the decisions neighboring this change, not just the file I edited.
• New or retitled decisions have a matching line in INDEX.md, and front matter follows CONTRIBUTING.md.
• python3 scripts/validate.py --base main passes locally, or I am relying on CI to tell me.
• No pricing, rates, market or customer allocation, consumer data, or client-confidential information.
• Claims about law or regulation are flagged for verification, not asserted.
• Commits are signed off (DCO). If I forgot, I will post the retroactive sign-off comment per CONTRIBUTING.md.
• I license this contribution under CC BY 4.0 (prose) and Apache 2.0 (embedded code samples).