fix(tools): enforce a disabled run_in_background at execution time (review findings)

enableRunInBackground: false removed the parameter from the advertised
schema only — the arg validator deliberately allows undeclared keys, so
a caller (or a model that has seen the parameter elsewhere) could still
force run_in_background: true and start background work past the
deployment's opt-out, in both tool-bash and tool-subagent. Both
producers now refuse the forced key loud in execute(); tests pin the
refusal (and that nothing spawns) alongside the untouched foreground
path; the schema-omission-is-advertising rule is recorded in the
runtime RFC and both READMEs.

docs/rfc/implemented/architecture/2026-06-20-generic-long-running-tool-runtime.md

@@ -100,7 +100,7 @@ Completion notices stay durable context, not a wake-up (`agent.inject()` appends
 
 ## Producer opt-in and schema exposure
 
-Whether a producer tool offers `run_in_background` is that producer's own defaulted config: `enableRunInBackground?: boolean` on `dsh-tool-bash` and on each `dsh-tool-subagent` instance (both default `true` — bash keeps its always-exposed behavior, and a deployment disables either per instance from cordis.yml, no code edit). A disabled producer omits the parameter from its schema entirely, so schema and capability can never disagree. `ctx.tasks` plays no part in schema shaping — it never rewrites or decorates a producer's tool schema (Kimi Code regex-rewrites its bash description when background is disabled; config-owns-the-schema makes that trick unnecessary) — it only provides runtime registration. The two halves compose fail-loud: the producer's config decides what the model sees, and a background call that still reaches `start()` without a control surface throws the load-this-package error. `start()` preflights every failable check (the fence, validation, the owner-cleanup attach) BEFORE invoking the producer's `run()` and commits atomically after — background work started without a collectable id is structurally impossible, not a producer rollback obligation.
+Whether a producer tool offers `run_in_background` is that producer's own defaulted config: `enableRunInBackground?: boolean` on `dsh-tool-bash` and on each `dsh-tool-subagent` instance (both default `true` — bash keeps its always-exposed behavior, and a deployment disables either per instance from cordis.yml, no code edit). A disabled producer omits the parameter from its schema entirely — and, because the arg validator deliberately allows undeclared keys, its `execute` ALSO refuses a forced `run_in_background: true` loud (the omission is advertising; the execution-time check is the enforcement). `ctx.tasks` plays no part in schema shaping — it never rewrites or decorates a producer's tool schema (Kimi Code regex-rewrites its bash description when background is disabled; config-owns-the-schema makes that trick unnecessary) — it only provides runtime registration. The two halves compose fail-loud: the producer's config decides what the model sees, and a background call that still reaches `start()` without a control surface throws the load-this-package error. `start()` preflights every failable check (the fence, validation, the owner-cleanup attach) BEFORE invoking the producer's `run()` and commits atomically after — background work started without a collectable id is structurally impossible, not a producer rollback obligation.
 
 ## The awaited owner-cleanup seam