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.

packages/core/agent-loop/tsconfig.json

@@ -38,6 +38,9 @@
     {
       "path": "../../core/scope"
     },
+    {
+      "path": "../../settings/settings"
+    },
     {
       "path": "../../support/invariants"
     }