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>

CHANGELOG.md

@@ -13,7 +13,7 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 - `codegraph init` now builds the initial index by default — you no longer need the `-i`/`--index` flag (it's still accepted, so existing commands and scripts keep working). (#483)
 - Go: Gin middleware chains now connect end-to-end in `codegraph_trace` and `codegraph_explore` — following a request reaches the middleware and route handlers registered via `.Use()` / `.GET()` instead of dead-ending where the framework dispatches the chain dynamically.
-- `codegraph_explore` is now leaner on interface-heavy flows: when a query spans many interchangeable implementations of one interface (an HTTP interceptor chain, say), it shows the rest as signatures instead of every full body, while keeping the dispatch mechanism and any specific method you asked about in full. Fewer tokens for the same answer, so questions like these stop costing more than plain grep/read — in testing, the two slowest-to-pay-off repos (a Java and a Python framework) went from slightly costlier than native search to clearly cheaper. Distinct, non-interchangeable code is shown in full as before. Disable with `CODEGRAPH_ADAPTIVE_EXPLORE=0`.
+- `codegraph_explore` now sizes its response to the *answer* instead of the file count: it shows the mechanism and the exact methods you asked about in full — even when they're buried deep in a large file — while collapsing the redundant interchangeable implementations of an interface (an HTTP interceptor chain, a query-compiler family) down to signatures. Fewer tokens for a more complete answer, so on the flows that used to occasionally cost more than plain grep/read it's now clearly cheaper — and the win holds across small, medium, and large codebases. Distinct, non-interchangeable code is shown in full as before. Disable with `CODEGRAPH_ADAPTIVE_EXPLORE=0`.
 
 ### Fixes