Add MCP support for Claude-compatible skill distributions

[snissn]

Summary

This PR adds distribution-aware MCP support for generated Claude-compatible skill bundles while preserving the existing Markdown-pack tools.

What changed

• kept existing tools intact:
  • agenthub_list
  • agenthub_versions
  • agenthub_fetch
  • agenthub_docs
• added new distribution-aware tools:
  • agenthub_distributions
  • agenthub_fetch_distribution
  • agenthub_fetch_distribution_file
• updated MCP docs output to describe both canonical packs and generated Claude-compatible skills
• updated the MCP tutorial with examples for listing and fetching Claude-compatible skill bundles
• updated Netlify function packaging to include distributions/**
• added a local MCP validation script that exercises the new tools against the function directly

Design choice

Per issue #13 PR 4 guidance, this PR uses new distribution-specific tools instead of overloading agenthub_fetch with new parameters. That keeps existing Markdown-pack clients working unchanged.

Validation

• npm run check:mcp-distributions
• npm run build
• direct handler spot-check of GET /mcp?format=json

Notes

• canonical Markdown pack retrieval remains unchanged
• this PR is stacked on Add website support for Claude-compatible skill distributions #16 because the MCP surface now reads the generated pilot bundles under distributions/claude/**
• the Milestone B Claude Code login blocker from Add pack-to-Claude-skill compiler and pilot outputs #15 still exists and is not resolved in this PR

Refs: #13