docs: README front door, contributing guide, and install skill

[alexdukeinvertase]

Applies the approved README and contributor front door. This account cannot push to invertase/react-native-coverage, so the branch is on the fork.

Changes

• README: one-line agent prompt to the app developers page, labelled human path, npm downloads and version badges, contributing links, Invertase org code of conduct, footer avatar and UTM.
• CONTRIBUTING.md: bugs and features via issues, Discord for questions, setup and test commands, and how an agent that is changing this repo should work.
• Install skill at .agents/skills/integrate-react-native-coverage/SKILL.md. It only reads the app developers page. .gitignore still ignores the rest of .agents/.

Left as they are

• Install command order (yarn add / npm install).
• Honeycomb hero.
• "Used in production by".
• Example and releasing sections in the README.
• No pull request template. Discussions links in the issue config are unchanged.
• AGENTS.md has no new banner.
• GitHub About, website, and topics are still empty. Setting them needs admin on the Invertase repo. Proposed text is in the project notes, not in this diff.

Install was not run. Codecov percentages in the README are unchanged from main and were not rechecked.