feat: wire agent.delegate tool with sub-agent configs

- Export createAgentDelegateTool through builtin/index.ts → tools/index.ts
- Register agent.delegate in routing.ts with lazy orchestrator pattern
- Add agent.delegate + agents.list to messaging and coding policy profiles
- Add group:agents tool group to policy.ts
- Add research/code/comms agent config examples to default.yaml
- Add research/code/comms agent configs to user config.yaml
- Add 11 tests for agent-delegate tool (all pass)
- Typecheck clean, no regressions

src/config/schema.ts

@@ -170,13 +170,30 @@ const modelsSchema = z.object({
 });
 
 const backendsSchema = z.object({
+  default: z.enum(['claude_code', 'opencode', 'codex', 'gemini']).optional(),
   claude_code: z.object({
     enabled: z.boolean().default(false),
     path: z.string().optional(),
+    args: z.array(z.string()).default([]),
+    timeout_ms: z.number().min(1_000).max(600_000).default(120_000),
   }).default({ enabled: false }),
   opencode: z.object({
     enabled: z.boolean().default(false),
     path: z.string().optional(),
+    args: z.array(z.string()).default([]),
+    timeout_ms: z.number().min(1_000).max(600_000).default(120_000),
+  }).default({ enabled: false }),
+  codex: z.object({
+    enabled: z.boolean().default(false),
+    path: z.string().optional(),
+    args: z.array(z.string()).default([]),
+    timeout_ms: z.number().min(1_000).max(600_000).default(120_000),
+  }).default({ enabled: false }),
+  gemini: z.object({
+    enabled: z.boolean().default(false),
+    path: z.string().optional(),
+    args: z.array(z.string()).default([]),
+    timeout_ms: z.number().min(1_000).max(600_000).default(120_000),
   }).default({ enabled: false }),
   native: z.object({
     enabled: z.boolean().default(true),
@@ -691,6 +708,7 @@ const sandboxSchema = z.object({
 const agentConfigEntrySchema = z.object({
   system_prompt: z.string().optional(),
   model_tier: modelTierEnum.optional(),
+  backend: z.enum(['native', 'claude_code', 'opencode', 'codex', 'gemini']).optional(),
   tool_profile: toolProfileEnum.optional(),
   tool_overrides: toolOverrideSchema.optional(),
   sandbox: z.boolean().default(false),