feat(store): persist subagent tasks to PostgreSQL (#600)

- Migration 000034: subagent_tasks table with tenant scope, JSONB
  metadata + GIN index, partial index for archival candidates
- SubagentTaskStore interface with Create/Get/UpdateStatus/List/Archive
- PG implementation with parameterized queries and tenant isolation
- SQLite schema v3→4 migration + no-op stub for Lite edition
- Wire into store.Stores and factories

internal/store/sqlitestore/schema.sql

@@ -1311,3 +1311,38 @@ CREATE TABLE IF NOT EXISTS system_configs (
 );
 
 CREATE INDEX IF NOT EXISTS idx_system_configs_tenant ON system_configs(tenant_id);
+
+-- ============================================================
+-- Table: subagent_tasks
+-- ============================================================
+
+CREATE TABLE IF NOT EXISTS subagent_tasks (
+    id                TEXT PRIMARY KEY,
+    tenant_id         TEXT NOT NULL REFERENCES tenants(id) ON DELETE CASCADE,
+    parent_agent_key  VARCHAR(255) NOT NULL,
+    session_key       VARCHAR(500),
+    subject           VARCHAR(255) NOT NULL,
+    description       TEXT NOT NULL,
+    status            VARCHAR(20) NOT NULL DEFAULT 'running',
+    result            TEXT,
+    depth             INTEGER NOT NULL DEFAULT 1,
+    model             VARCHAR(255),
+    provider          VARCHAR(255),
+    iterations        INTEGER NOT NULL DEFAULT 0,
+    input_tokens      INTEGER NOT NULL DEFAULT 0,
+    output_tokens     INTEGER NOT NULL DEFAULT 0,
+    origin_channel    VARCHAR(50),
+    origin_chat_id    VARCHAR(255),
+    origin_peer_kind  VARCHAR(20),
+    origin_user_id    VARCHAR(255),
+    spawned_by        TEXT,
+    completed_at      TEXT,
+    archived_at       TEXT,
+    metadata          TEXT NOT NULL DEFAULT '{}',
+    created_at        TEXT DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
+    updated_at        TEXT DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_subagent_tasks_parent_status ON subagent_tasks(tenant_id, parent_agent_key, status);
+CREATE INDEX IF NOT EXISTS idx_subagent_tasks_session ON subagent_tasks(session_key);
+CREATE INDEX IF NOT EXISTS idx_subagent_tasks_created ON subagent_tasks(tenant_id, created_at);