docs: add Plumber Score badge to README

[thomasboni]

Adds the live Plumber Score badge to the README, now that score publishing is enabled (PR #47).

[![Plumber Score](https://score.getplumber.io/github.com/getplumber/platform.svg)](https://score.getplumber.io/github.com/getplumber/platform)

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
README-only change with no runtime, security, or data impact.

Overview
Adds a Plumber Score badge at the top of README.md, linking to the live score page for getplumber/platform on score.getplumber.io. It sits alongside the existing CI and Release badges.

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