Fragility-weighted severity: read Phoenix fragility scores for impact weighting

[CodeBuildder]

Context

Argus upweights threat severity when the targeted entity also has a high Phoenix fragility score. Parent: #84.

Acceptance Criteria

• Reasoning agent reads GET /fragility?entity_id=X from world model for each threat target
• Final severity = base_severity × fragility_multiplier (formula documented in code comments)
• Threats on high-fragility entities promoted to next severity tier
• Integration test: inject threat on entity with fragility=0.9 → severity escalated in world model

Dependencies

Blocked by: none | Blocks: none

Notes

No model tier. Reads world model fragility API.