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.

pnpm-lock.yaml

@@ -3301,6 +3301,9 @@ importers:
       '@deepseek-ai/dsh-session-persistence-jsonl':
         specifier: workspace:^
         version: link:../../session/session-persistence-jsonl
+      '@deepseek-ai/dsh-settings':
+        specifier: workspace:^
+        version: link:../../settings/settings
       '@deepseek-ai/dsh-system-prompt':
         specifier: workspace:^
         version: link:../system-prompt