feat(mcp): per-symbol adaptive codegraph_explore sizing (#569)

Sizes codegraph_explore to the answer, not the file count: shows the mechanism +
the exact methods you named in full (even buried in a large file) while collapsing
redundant interchangeable implementations to signatures. Adds uniqueness-aware
spare, per-symbol focused rendering of family files, all-tier test-file exclusion,
and named-method cluster survival in non-sibling god-files.

Validated A/B (Opus 4.8, 7-repo sweep): avg 25%% cheaper / 57%% fewer tokens / 23%%
faster / 62%% fewer tool calls. Django 9->23%% cheaper (0 reads), OkHttp 4->11%%
cheaper; gains across small/medium/large, inert repos unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>