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>