feat(commands): add /subagents spawn command

Add a `spawn` action to the /subagents command handler that invokes
spawnSubagentDirect() to deterministically launch a named subagent.

Usage: /subagents spawn <agentId> <task> [--model <model>] [--thinking <level>]

Also includes the shared subagent-spawn module extraction (same as the
refactor/extract-shared-subagent-spawn branch) since it hasn't merged yet.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/auto-reply/reply/commands-spawn.test-harness.ts

@@ -0,0 +1,46 @@
+import type { OpenClawConfig } from "../../config/config.js";
+import type { MsgContext } from "../templating.js";
+import { buildCommandContext } from "./commands-context.js";
+import { parseInlineDirectives } from "./directive-handling.js";
+
+export function buildCommandTestParams(
+  commandBody: string,
+  cfg: OpenClawConfig,
+  ctxOverrides?: Partial<MsgContext>,
+) {
+  const ctx = {
+    Body: commandBody,
+    CommandBody: commandBody,
+    CommandSource: "text",
+    CommandAuthorized: true,
+    Provider: "whatsapp",
+    Surface: "whatsapp",
+    ...ctxOverrides,
+  } as MsgContext;
+
+  const command = buildCommandContext({
+    ctx,
+    cfg,
+    isGroup: false,
+    triggerBodyNormalized: commandBody.trim().toLowerCase(),
+    commandAuthorized: true,
+  });
+
+  return {
+    ctx,
+    cfg,
+    command,
+    directives: parseInlineDirectives(commandBody),
+    elevated: { enabled: true, allowed: true, failures: [] },
+    sessionKey: "agent:main:main",
+    workspaceDir: "/tmp",
+    defaultGroupActivation: () => "mention",
+    resolvedVerboseLevel: "off" as const,
+    resolvedReasoningLevel: "off" as const,
+    resolveDefaultThinkingLevel: async () => undefined,
+    provider: "whatsapp",
+    model: "test-model",
+    contextTokens: 0,
+    isGroup: false,
+  };
+}