Add integration tests for discovery system

[kivo360]

Problem

The discovery system — which enables adaptive workflow branching when agents find new work — has no integration tests:

• DiscoveryService — Core discovery tracking and task spawning
• DiscoveryAnalyzerService — LLM-powered pattern analysis and blocker prediction

What to do

Create backend/tests/integration/test_discovery_integration.py:

1. Test discovery creation during task execution
2. Test discovery-based task spawning (branch tasks bypass normal phase order)
3. Test the Hephaestus pattern: Phase 3 agent spawns Phase 1 investigation tasks
4. Test DiscoveryAnalyzerService pattern recognition with mock LLM

Reference: docs/architecture/03-discovery-system.md

Difficulty: Medium | Skill: Python (pytest, async)