New src/core/code-graph-readiness.ts: resolveCodeReadiness() returns a typed
status (not_built | indexing | ready | unknown) + ready boolean so callers can
tell "graph not built / still indexing" apart from "genuinely no match" when
count===0. EXISTS-based (cheap), chunk-grain, resolver-version-matching pending
predicate, fail-open. Wired into the 4 CLI envelopes (+ human hint) and the 4
MCP op handlers. def/refs are 2-state brain-wide; callers/callees 3-state scoped.
