Task: Regression detection and diagnostic reasoning

Description

Implement the regression detection algorithm that compares each response's score against rolling averages and flags quality drops. Includes severity classification (minor/moderate/severe) and diagnostic reasoning that explains why a score was low, correlating sub-score drops with observable signals.

Acceptance Criteria

• packages/opencode/src/quality/detector.ts created
• Fetches rolling window (last N responses, configurable via regression.windowSize) from SQLite
• Computes rolling average for composite score and each sub-score
• Calculates delta: current_score - rolling_average
• Severity classification: minor (-5 to -15), moderate (-15 to -30), severe (below -30)
• Regressions persisted to regressions table with: response_id, score_delta, severity, component, description
• Diagnostic reasoning generated for each regression: identifies which sub-scores dropped, correlates with signals (e.g., "3 new type errors", "user reverted change", "tool invocation failed")
• Emits Quality.Event.Regression event on bus when severity >= configured alertLevel
• Handles edge cases: fewer than windowSize responses (use available data), first response (no comparison, no regression)
• Regression detection runs as part of the scoring pipeline (after score computed, before response to user)

Technical Details

• Define Quality.Event.Regression using BusEvent.define() pattern
• Query rolling averages from SQLite: SELECT AVG(composite_score), AVG(correctness_score), AVG(feedback_score), AVG(analysis_score) FROM responses ORDER BY timestamp DESC LIMIT ?
• Diagnostic reasoning uses a template system: check each sub-score delta, match to known signal patterns, compose human-readable explanation
• Regression event payload: { responseId, severity, delta, component, description } — TUI can subscribe to show inline warnings

Dependencies

• Task 001: SQLite storage layer (for queries)
• Task 003: Scoring engine (provides scores to compare)

Effort Estimate

• Size: M
• Hours: 6-8
• Parallel: false (depends on 001, 003)