Expose risk weights and decision thresholds in YAML config

[riccardomerenda]

Problem

Key decision parameters are hardcoded in decision.py:

• schema_break_rate hard stop at 0.25
• high_criticality_failure_rate hard stop at 0.20
• proceed_if_overall_risk_below at 0.08
• High-criticality weight at 2.0x
• Latency thresholds (p50: 1.5x, p95: 2.0x)

Users with different risk tolerances (e.g., internal tools vs. customer-facing APIs) can't tune these without editing source code.

Proposal

Add an optional decision section to the YAML config:

decision:
  hard_stop_schema_break_rate: 0.25
  hard_stop_high_crit_failure_rate: 0.20
  proceed_risk_threshold: 0.08
  high_criticality_weight: 2.0
  latency_p50_threshold: 1.5
  latency_p95_threshold: 2.0

All fields optional with current values as defaults — zero breaking changes.

Impact

• Users can tune sensitivity per migration
• Enables stricter thresholds for critical systems, looser for experimentation