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/module-graph.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/module-graph.md
-module-graph.md: 65741283b5a48817bd01c42fe7982fff6c89511a
-module-graph.zh.md: 00ea2925c6636a65e224c899ee264a4e87d4a2cd
+module-graph.md: 949f8813f1d8f49e5a433049eb2f61c7235bf571
+module-graph.zh.md: 70085ab7301cf3ec81494189aea065c9a487b04c