feat: GitHub integration plugin with bidirectional sync

[om952]

• Implemented GitHub integration plugin (packages/plugins/github/) with bidirectional issue sync between Paperclip and GitHub
• Added plugin services for creating, updating, and linking GitHub issues from Paperclip tasks
• Integrated plugin into the workspace build system with proper TypeScript configuration and exports
• Added comprehensive README with setup instructions, configuration reference, and usage examples
• Added unit tests for plugin services and integration tests for the full sync flow
• Added architecture documentation explaining the plugin's design, data flow, and extension points
• Fixed workspace integration issues to ensure plugin compiles and tests pass alongside core packages

Verification

# Install dependencies
pnpm install

# Run the plugin tests
pnpm test

# Run full typecheck to ensure no regressions
pnpm -r typecheck

# Build the project
pnpm build

Manual verification steps:

1. Configure a GitHub personal access token in the plugin settings
2. Link a Paperclip issue to a GitHub repository
3. Verify issue title/body syncs to GitHub
4. Update the issue in GitHub and confirm changes reflect back in Paperclip

Risks

• Low to medium. The plugin is additive and does not modify core Paperclip logic, but the bidirectional sync introduces potential for race conditions if the same issue is edited concurrently in both systems.
• The plugin requires a valid GitHub PAT with appropriate repo scopes; misconfigured tokens will fail gracefully with error logging but users need documentation to diagnose.
• Workspace integration changes could theoretically affect other packages, but the fix was scoped to the plugin's own build configuration.

Closes #15