De-fork spike S2: execution-route inventory + sentinel tests

[anandgupta42]

Stage S2 (spec docs/internal/2026-07-18-defork-spike-spec.md). Authoritative map of every tool-execution dispatcher (session/tools.ts resolver, parallel session/prompt.ts resolver, batch.ts direct dispatch, Task path, MCP wrapper) separated from ingress surfaces; each active route gets an executable sentinel test. Feeds S3 HardPolicy insertion points. Deliverables: docs/internal/defork-route-matrix.md + packages/opencode/test/altimate/defork/route-sentinels.test.ts. Analysis + tests only, zero product changes.