feat(plugins): add GitHub integration plugin

[patkaryash]

Summary

This PR introduces a new GitHub Integration plugin for Paperclip.

The plugin listens for Paperclip issue creation events and automatically creates the corresponding issue in a configured GitHub repository. It also records synchronization metadata to prevent losing the relationship between Paperclip and GitHub issues.

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Thinking Path

• Paperclip supports integrations through its plugin system.
• Many teams track work in GitHub alongside Paperclip.
• Previously, creating a Paperclip issue required manually creating the corresponding GitHub issue.
• This duplicated work and increased the chance of the two systems becoming inconsistent.
• This PR introduces a dedicated GitHub integration plugin that automatically synchronizes new Paperclip issues to GitHub.
• The result is a cleaner workflow and a foundation for future bidirectional synchronization.

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What Changed

• Added a new @paperclipai/plugin-github-integration plugin package.
• Added a plugin manifest describing capabilities and configuration.
• Registered a worker that subscribes to issue.created events.
• Implemented GitHub Issue creation using the GitHub REST API.
• Added configurable repository owner, repository name, and authentication settings.
• Stored synchronization metadata using the plugin state API.
• Logged synchronization events through the Paperclip activity API.
• Added package configuration, TypeScript configuration, and plugin documentation.

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Verification

• Built and loaded the plugin successfully.
• Verified plugin startup and event subscription.
• Confirmed issue.created events are received.
• Verified GitHub issues are created successfully in the configured repository.
• Confirmed synchronization state is stored after successful issue creation.
• Verified activity entries are written for successful synchronizations.

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Risks

• Low risk.
• The plugin only acts on issue.created events and does not modify existing GitHub issues.
• Successful operation depends on a valid GitHub Personal Access Token and repository configuration.
• This PR establishes one-way synchronization (Paperclip → GitHub). Bidirectional synchronization is outside the scope of this change.

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Model Used

• Hermes CLI (Kimi Coding) for implementation assistance.
• ChatGPT GPT-5.5 for architecture review and PR preparation.

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Checklist

• I have included a thinking path that traces from project context to this change
• I have specified the model used
• I have checked ROADMAP.md and confirmed this work does not duplicate planned core work
• I have run tests locally
• I have added or updated documentation where appropriate
• I have considered and documented the risks