feat: Add task_delegations schema (Session 374)

- task_delegations table for tracking delegation metadata per task
- task_learning_effectiveness table for per-task learning tracking
- Indexes for efficient querying by task_id, status, backend

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/002_task_delegations.sql

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+-- Migration 002: Task-Delegation Integration
+-- Session 374: Unified task management with code delegation
+
+-- Task delegation tracking table
+CREATE TABLE IF NOT EXISTS task_delegations (
+    id SERIAL PRIMARY KEY,
+    task_id TEXT NOT NULL REFERENCES tasks(id) ON DELETE CASCADE,
+    delegation_id TEXT NOT NULL UNIQUE,
+    backend TEXT NOT NULL,
+    quality_score INTEGER,
+    learnings_injected TEXT,  -- Comma-separated learning IDs
+    status TEXT DEFAULT 'pending' CHECK (status IN ('pending', 'success', 'failed', 'partial')),
+    started_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    completed_at TIMESTAMP WITH TIME ZONE,
+    input_tokens INTEGER DEFAULT 0,
+    output_tokens INTEGER DEFAULT 0,
+    cost_usd NUMERIC(10,6) DEFAULT 0,
+    context_hash TEXT,  -- MD5 of task file for dedup
+    error_message TEXT
+);
+
+CREATE INDEX IF NOT EXISTS idx_task_delegations_task ON task_delegations(task_id);
+CREATE INDEX IF NOT EXISTS idx_task_delegations_status ON task_delegations(status);
+CREATE INDEX IF NOT EXISTS idx_task_delegations_backend ON task_delegations(backend);
+CREATE INDEX IF NOT EXISTS idx_task_delegations_started ON task_delegations(started_at);
+
+-- Per-task learning effectiveness tracking
+CREATE TABLE IF NOT EXISTS task_learning_effectiveness (
+    id SERIAL PRIMARY KEY,
+    task_id TEXT NOT NULL REFERENCES tasks(id) ON DELETE CASCADE,
+    learning_id INTEGER NOT NULL,
+    outcome TEXT NOT NULL CHECK (outcome IN ('success', 'failed')),
+    recorded_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_task_learning_task ON task_learning_effectiveness(task_id);
+CREATE INDEX IF NOT EXISTS idx_task_learning_learning ON task_learning_effectiveness(learning_id);
+CREATE INDEX IF NOT EXISTS idx_task_learning_outcome ON task_learning_effectiveness(outcome);
+
+-- Migration tracking
+INSERT INTO schema_migrations (version, applied_at) VALUES ('002_task_delegations', NOW())
+ON CONFLICT DO NOTHING;