Add GH Project plugin

[zfifteen]

Summary

• Add GH Project to the Tools & Integrations community plugin list.
• Keep the PR scoped to the README entry requested by CONTRIBUTING.md.

Requirement Check

• Public repo: https://github.com/zfifteen/gh-project-plugin
• Manifest: .codex-plugin/plugin.json present and valid.
• Functional release: v1.0.0 published with packaged release asset.
• Description: one-line concise catalog description.
• Author link: entry links to the zfifteen GitHub repo.
• Alphabetical order: python3 scripts/check-alphabetical.py passes.

Validation

• uvx codex-plugin-scanner verify . passes on zfifteen/gh-project-plugin.
• uvx codex-plugin-scanner lint . reports policy_pass=True with an effective score of 84.
• npm run validate passes in the plugin repo and confirms the production package surface.

[gemini-code-assist]

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[Copilot]

Pull request overview

Adds a single entry for the "GH Project" community plugin to the Tools & Integrations list in the README, maintaining alphabetical order.

Changes:

• Insert a new bullet for GH Project linking to zfifteen/gh-project-plugin with a one-line description.

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