review: acquire the structured runtime per run, not per backend

The Codex simplification concern plus the duplication comment on the spawn
apply, resolved by deletion: the backend-lifetime holds are gone, so the
runtime registers at the first structured run and disposes when the last
settles — a deployment that never passes outputSchema carries no always-on
global state, and there is no per-backend acquisition block left to extract.

The driver spec now drives an INLINE spawn-shaped provider over
startInProcessRun, which removes the spawn/fork devDependencies (the
test-only workspace cycle); plugin-level structured coverage moves to the
backends' own specs (capture through the shipped plugin, mid-run backend
unload, seeded fork capture). tools.md, the driver README, and both backend
READMEs describe the run-scoped lifetime; the module-graph regenerates
without the cycle edges.

pnpm-lock.yaml

@@ -655,12 +655,6 @@ importers:
       '@deepseek-ai/dsh-subagent':
         specifier: workspace:^
         version: link:../subagent
-      '@deepseek-ai/dsh-subagent-fork':
-        specifier: workspace:^
-        version: link:../subagent-fork
-      '@deepseek-ai/dsh-subagent-spawn':
-        specifier: workspace:^
-        version: link:../subagent-spawn
       '@deepseek-ai/dsh-system-prompt':
         specifier: workspace:^
         version: link:../../core/system-prompt