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/config-catalog.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/config-catalog.md
-config-catalog.md: 14610ca9c06d5c4beb6ccb74795e5859952c58a4
+config-catalog.md: b588a80d6ee773fde52d9c3050b9d76438810d02
 config-catalog.zh.md: d092947d31cfe4b24cae5d0ee8570dda39d7a287