feat(mcp): adaptive codegraph_explore sizing — skeletonize redundant polymorphic siblings (#564)

codegraph_explore now skeletonizes off-spine, redundant members of a polymorphic
family (OkHttp's interceptor chain, Django's SQLCompiler family) to signatures
instead of shipping every full body, while keeping the dispatch mechanism, the
orchestrator/base, and any method the agent named in full. Sizes the response to
the answer rather than the budget cap, so interface-heavy flows stop costing more
than plain grep/read. Default on; CODEGRAPH_ADAPTIVE_EXPLORE=0 disables.

Gate: off-spine + >=3-impl sibling + not-spared, where spared = the agent named a
callable in the file UNLESS the file defines the family's supertype (a huge
base+subclasses file is Read-anyway, so skeletonizing frees explore budget).

Validated headless A/B (Opus 4.8): both former README cost outliers flipped —
OkHttp and Django went from costlier-than-native to cheaper; full 7-repo average
22%% cheaper / 47%% fewer tokens / 20%% faster / 50%% fewer tool calls, every repo
cost-positive, inert repos unchanged. 7-case regression test.

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

CHANGELOG.md

@@ -13,6 +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`.
 
 ### Fixes