feat(mcp): size-adaptive output budget for codegraph_explore (#185) (#187)

Output is now scaled to indexed file count. Small projects (<500 files)
cap at ~18KB and skip the "Additional relevant files" / completeness /
explore-budget reminders that earn their keep on larger codebases; medium
(<5,000) caps at ~28KB; large (<15,000) keeps the historical ~35KB; very
large goes up to ~38KB.

A per-file char cap also prevents a single file with many adjacent
symbols from collapsing into one whole-file dump (the pathological
Alamofire `Session.swift` case reported in #185), and a per-file symbol-
list cap stops the `#### path — sym(kind), ...` header from leaking
multi-KB lists when many adjacent symbols cluster together.

Measured against the README's benchmark repos: Alamofire (~100 files)
~62% smaller per call, Excalidraw (~600 files) ~35%, VS Code (~10k
files) ~14%. Agent-trust floor preserved — Relationships, scored cluster
selection, and structured-source output are all retained.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>