feat(agent-loop): make the parallel tool-call cap a user setting

The section is a strict subset of the plugin config: `agents` is consumed
once when the service starts, so a stored change there could only look like
it had an effect. The cap resolves through a getter over the settings source,
which the scheduler destructures at the start of each tool group, so a
committed change bounds the next group without disturbing the one in flight.
`resolveMaxParallelToolCalls` becomes the section validator, refusing a value
at the write instead of at that group.

The deferred-resume effect-shape assertion now allows the one plugin effect
the optional settings wiring adds at the fiber's own level; a resumed agent
joining it there is still the regression it pins.

docs/event-producer-consumer.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write docs/event-producer-consumer.md
-event-producer-consumer.md: 622df039a42bccb1ecf8dcaf11a1c68ea111dec1
+event-producer-consumer.md: b9f8fbaf92e1d3da99bd233496c717d66a0f55e6
 event-producer-consumer.zh.md: 976f41d7798e9182b60366d77e546d79c4a66a13