docs: add user guide and test plan

[windoliver]

Summary

• add a use-case-driven user guide covering CLI, TUI, MCP, server, Nexus, GitHub, gossip, and ask-user workflows
• add a package-by-package test plan with coverage gaps, priorities, and a manual TUI checklist
• link both docs from the README for discoverability

Validation

• git push pre-push hook
  • bun run typecheck
  • bun run build