feat: complete phase 2 -- multi-agent routing, interrupt TTL, escalation, templates

- Intent classification with LLM structured output (single/multi/ambiguous)
- Discount agent with apply_discount and generate_coupon tools
- Interrupt manager with 30-min TTL auto-expiration and retry prompts
- Webhook escalation module with exponential backoff retry (max 3)
- Three vertical industry templates (e-commerce, SaaS, fintech)
- Template loading in AgentRegistry
- Enhanced supervisor prompt with dynamic agent descriptions
- 153 tests passing, 90.18% coverage

backend/pyproject.toml

@@ -18,6 +18,7 @@ dependencies = [
     "pydantic-settings>=2.7,<3.0",
     "pyyaml>=6.0,<7.0",
     "python-dotenv>=1.0,<2.0",
+    "httpx>=0.28,<1.0",
 ]
 
 [project.optional-dependencies]