feat(container-config): add per-group model + effort overrides

Allow individual agent groups to opt into different models or effort levels
without changing host-wide defaults. Useful when one group is high-stakes
(opus, high effort) but most are routine (sonnet/haiku, low effort).

container.json gains two optional fields:
  - model: alias ("sonnet" | "opus" | "haiku") or full model ID
  - effort: "low" | "medium" | "high" | "xhigh" | "max"

Both omitted = SDK default (current behavior). The host plumbs them as
NANOCLAW_MODEL / NANOCLAW_EFFORT env vars at container spawn time; the
agent-runner reads them in providers/index.ts and threads through to the
provider via ProviderOptions. The Claude provider passes them straight to
sdkQuery options.

`effort` is currently typed as `any` because the @anthropic-ai/claude-
agent-sdk type doesn't surface it yet — passing it through still works at
runtime via the SDK's loose option handling. Drop the cast once the SDK
adds an `effort` field to its options type.

container/agent-runner/src/config.ts

@@ -16,6 +16,8 @@ export interface RunnerConfig {
   agentGroupId: string;
   maxMessagesPerPrompt: number;
   mcpServers: Record<string, { command: string; args: string[]; env: Record<string, string> }>;
+  model?: string;
+  effort?: string;
 }
 
 const DEFAULT_MAX_MESSAGES = 10;
@@ -43,6 +45,8 @@ export function loadConfig(): RunnerConfig {
     agentGroupId: (raw.agentGroupId as string) || '',
     maxMessagesPerPrompt: (raw.maxMessagesPerPrompt as number) || DEFAULT_MAX_MESSAGES,
     mcpServers: (raw.mcpServers as RunnerConfig['mcpServers']) || {},
+    model: (raw.model as string) || undefined,
+    effort: (raw.effort as string) || undefined,
   };
 
   return _config;