docs: align README and contributing guide with the sibling plugins

[milanagm]

Aligns the README with the sibling observability plugins for opencode and Claude Code, in structure and in information density (19 KB down to 3.7 KB).

• README follows opencode's sections: Quick Start, Supported Versions, Langfuse Credentials, Contributing, License.
• Adds CONTRIBUTING.md adapted from opencode's, keeping the Codex specifics that both manifest versions have to be bumped and that .agents/plugins/marketplace.json has to be repointed after a release.
• Links the Langfuse Codex integration page.

[claude]

Looks good — a docs-only restructuring that's accurate. Checked the new CONTRIBUTING.md commands (build/test/format/lint) against the actual package.json scripts — they match. Verified the referenced paths (plugins/tracing/package.json, plugins/tracing/.codex-plugin/plugin.json, .agents/plugins/marketplace.json) exist. The condensed README content (versions, env resolution order, regions) is consistent with the detail it replaced.

Extended reasoning...

This PR only touches README.md and CONTRIBUTING.md (no code, config, or security-sensitive surface). It condenses README content into a new CONTRIBUTING.md and a shorter README, and I cross-checked the new build/test/lint commands against package.json scripts and the referenced file paths (package.json, plugin.json, marketplace.json), all of which exist and match. The change is mechanical documentation restructuring with no ambiguity or design decisions requiring human input.