Workgraph init onboarding

[G9Pedro]

Rebuild workgraph init to provide an idempotent, fully-featured starter kit with guided first-agent registration.

The previous init command was minimal and not idempotent, leading to a fragmented and manual onboarding experience. This PR transforms workgraph init into a comprehensive setup that seeds all required default primitives (roles, policies, gates, space) as editable markdown files, generates a sensible server configuration, and includes a QUICKSTART.md to guide users through starting the server and registering their first agent via the new workgraph agent register command.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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