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/subsystems/core.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/subsystems/core.md
-core.md: af27484160769156836f377e5b3aba2521280005
-core.zh.md: 12935f4d881f371cfe2c3c5bed85ef88f57ec71a
+core.md: a900192f4cee2b53bbf4d2f6d00d9586d87d438a
+core.zh.md: 17ad5f03b34b8acff37689c38ed4323096774c0a