Nightwatch stagnation detection and redirection (Phase 4)

[AlexMikhalev]

Summary

Extend Nightwatch drift detection with stagnation metrics and automatic redirection strategies when agents stop making progress.

Approach

Stagnation Detection

• Extend DriftMetrics with novelty_score, learning_velocity, quality_trend
• Add observe_evolution() method and StagnationLevel classification
• Detect stagnation: 3+ consecutive windows with no new lessons

Redirection Strategies (tried in order)

1. Prompt variation: Append "Previous approaches exhausted. Try different angle: [prior approach summary]"
2. Model rotation: Force different model via RoutingEngine override
3. Task decomposition: Split into sub-tasks, hand off to fresh agents

Safeguards

• Require 3 consecutive stagnant windows before first redirect
• Cap at 3 redirections per agent per day
• All redirections logged as Process lessons

Critical Files

• crates/terraphim_orchestrator/src/nightwatch.rs -- StagnationMetrics, observe_evolution(), StagnationLevel
• crates/terraphim_orchestrator/src/lib.rs -- handle CorrectionAction::RedirectStrategy

Acceptance Criteria

• Nightwatch detects stagnation when no new lessons for 3 windows
• Prompt variation applied on first alert
• Model rotation on persistent stagnation
• All redirections logged
• cargo test --workspace passes

Dependencies

• Blocked by: Phase 1 (can run in parallel with Phases 2+3)
• Related: Phase 3: Success-rate monitoring and self-healing procedures #695 (success-rate monitoring and self-healing)