Add skill logo, Cursor plugin config, and Codex manifest

Bring the repo to parity with the SwiftUI-Agent-Skill reference:
- Add assets/logo.svg skill icon
- Add .cursor-plugin/plugin.json for Cursor marketplace submission
- Add agents/openai.yaml for Codex / OpenAI-compatible tools
- Update .claude-plugin/marketplace.json schema URL to full GitHub raw URL
- Update README with Cursor and Codex installation options and skill structure tree

agents/openai.yaml

@@ -0,0 +1,32 @@
+name: xcode-build-skills
+version: "1.0.0"
+description: >-
+  A multi-skill plugin for benchmarking and optimizing Xcode builds,
+  compile hotspots, project settings, and Swift Package Manager
+  dependency overhead.
+author:
+  name: Antoine van der Lee
+  email: contact@avanderlee.com
+repository: https://github.com/AvdLee/Xcode-Build-Optimization-Agent-Skill
+license: MIT
+logo: assets/logo.svg
+keywords:
+  - xcode
+  - builds
+  - incremental-builds
+  - clean-builds
+  - swift
+  - ios
+  - macos
+  - spm
+  - benchmarking
+  - compilation
+  - project-settings
+  - explicit-modules
+skills:
+  - skills/xcode-build-benchmark
+  - skills/xcode-compilation-analyzer
+  - skills/xcode-project-analyzer
+  - skills/spm-build-analysis
+  - skills/xcode-build-orchestrator
+  - skills/xcode-build-fixer