fix(serve): show community name in get_community MCP output

[rmart1308]

What

get_community was the only MCP graph query tool still returning a bare numeric
community id. get_node (and the query-traversal output in _subgraph_to_text)
already read the community_name node attribute that to_json writes onto every
node, so an agent that had located a community by name — via GRAPH_REPORT.md,
get_node, or query output — had no way to confirm it through get_community.

This makes get_community consistent with the rest of the serve layer: it reads
the name from the community's member nodes (they all share it) and formats it into
the header, e.g.

Community 12 — Auth & Sessions (8 nodes):
  login() [auth/router.py]
  ...

It falls back to the bare id for graphs built before community_name was
persisted, and routes the value through sanitize_label like every other
LLM-derived field (F-010).

Why

Small consistency/UX gap. Once you can see community names everywhere else, the
one tool whose whole job is "show me this community" shouldn't be the one that
hides them behind a number.

Notes

• Follows the existing get_node precedent (inline read of community_name), so
  no new test was added — the tool closures aren't unit-tested directly in
  tests/test_serve.py, and the community_name attribute itself is produced and
  tested in the to_json / export path. Happy to extract a small module-level
  formatter and add a focused test if you'd prefer the change be directly covered.
• CHANGELOG Unreleased entry uses a (#XXXX) placeholder — replace with the PR
  number on merge.

🤖 Generated with Claude Code

[safishamsi]

Landed on v8 (commit f9ded63), authored to you. I added two things on top of your change: the name is skipped when it is only the Community N placeholder (written for unnamed communities), so the header doesn't read Community 12 — Community 12; and I pulled the formatting into a small module-level _community_header() with focused tests (named / placeholder / empty / sanitised), which also answers your note about the tool closures not being directly testable. Verified: serve/MCP suite green, full suite 2397 passed. Thanks for spotting the consistency gap and for the clean writeup. Closing as landed.