Add configurable severity thresholds and custom rule profiles

[rudra496]

Summary

Allow users to create custom rule profiles with configurable severity thresholds.

What to implement

1. Rule Profiles

# .aitrust.yaml
profiles:
  strict:
    min_score: 90
    fail_on: [critical, high, medium]
  standard:
    min_score: 70
    fail_on: [critical, high]
  relaxed:
    min_score: 50
    fail_on: [critical]

2. CLI Usage

aitrust validate src/ --profile strict
aitrust validate src/ --profile standard   # default

3. Per-Category Thresholds

thresholds:
  security:
    fail_below: 80    # Security must score 80+
  hallucination:
    fail_below: 70
  logic:
    fail_below: 60
  best_practices:
    fail_below: 50

4. Custom Severity Override

overrides:
  # Promote bare-except from medium to high in this project
  bare_except: high
  # Demote mutable-default from medium to low
  mutable_default: low

Acceptance Criteria

• Built-in profiles: strict, standard, relaxed
• Custom profiles via config file
• Per-category thresholds work
• Severity overrides work
• Tests

Difficulty

Beginner — configuration handling, the validation engine already supports scoring.