Update onboarding and contributor docs for dual distribution

[snissn]

Summary

This PR covers issue #13 PR 5: documentation, onboarding, and contributor workflow updates for the canonical-pack plus generated-skill model.

What changed

• rewrote README.md around the dual-distribution model
• updated CONTRIBUTING.md to make the canonical-pack-first rule explicit
• added a dedicated tutorial:
  • tutorials/use-agent-hub-claude-compatible-skills.md
• updated the MCP tutorial to explain canonical packs vs generated Claude-compatible skills
• updated the onboarding prompt so the installed routing note teaches agents when MCP is the right path and when a local Claude-compatible skill is a better fit

Acceptance criteria covered

• README is current
• CONTRIBUTING is current
• at least one tutorial explains the dual-distribution model
• contributor expectations are unambiguous
• onboarding copy stays coherent after the distribution expansion

Validation

• npm run check:mcp-distributions
• npm run build

Notes

• this PR is stacked on Add MCP support for Claude-compatible skill distributions #17 because the new docs reference the distribution-aware MCP tools added there
• the Milestone B real-Claude validation blocker from Add pack-to-Claude-skill compiler and pilot outputs #15 still remains open and is not resolved here

Refs: #13