Remove the directory-community feature and its MCP tools

Module cohesion (community) was a coarse directory-based rollup with no
effect on retrieval or relation lookup (edges + ref annotations already
cover those), and it was the weakest-value feature per its cost. Remove:

- internal/analysis/community and the CommunityBuilder DI/interface.
- The community rebuild steps in build_or_update_graph and run_postprocess.
- The list_communities, get_community, and get_architecture_overview MCP
  tools plus their handler_graph handlers and response types.
- The community entries in get_minimal_context's tool suggestions.

run_postprocess keeps communities_count (always 0) for wire compatibility.
MCP tools: 31 -> 28. Full suite green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: tae2089