obs: blast-radius query — file/symbol to dependents, owning gates, and live-coverage owners #1425

Part of #1412 (Track C). Depends on #1410 (depgraph model).

Context

The question agents ask before every risky edit is "who depends on this and which tests own it" — today that is four separate lookups (depgraph, check-affected, iOS coverage manifest, ADR 0011 guarantee matrix).

Deliverables

• pnpm depgraph affected <path> (start file-level; symbol-level optional follow-up): reverse-reachability over the value-edge graph from scripts/depgraph/model.ts, reporting: direct/transitive dependents (with counts and zone breakdown), the check-affected gate plan for the dependent set, owning live scenarios from test/integration/ios-simulator-e2e/coverage-manifest.ts for any public command whose handler chain includes the file, and guarantee-matrix rows touching it.
• Compact text output for agent context budgets (bounded lists + counts); --json for tooling.
• One docs/agents section: "before editing a shared module, run this; the output tells you which gates to run and which live scenarios claim the behavior."

Acceptance

• Answers for a hot module (e.g. src/utils/exec.ts, src/daemon/ref-frame.ts) in < 10s.
• Output cross-checked against check-affected for at least three sample diffs (no contradictions).