fix(tasks): route updateTaskSession/getTaskSession through task_sessions table

Stop writing new session data back to the legacy tasks.session_json
column; updateTaskSession now upserts the active task_sessions row
(updating the session pointed to by tasks.active_session_id, creating
one when absent), and getTaskSession reads from the same row so
session data has a single source of truth. Adds migration 27 to
backfill pre-existing tasks.session_json values into task_sessions
idempotently via INSERT OR IGNORE guarded on task_id. The legacy
column is kept for read-only history and is not dropped.

src/main/database/schema.ts

@@ -365,6 +365,48 @@ export function runMigrations(
       version: 26, name: "add_source_run_id_for_causal_binding",
       sql: `ALTER TABLE documents ADD COLUMN source_run_id TEXT; ALTER TABLE knowledge_base ADD COLUMN source_run_id TEXT`,
     },
+    {
+      version: 27, name: "backfill_task_sessions_from_tasks_session_json",
+      fn: () => {
+        const legacyTasks = readAll<{
+          id: string;
+          companyId: string;
+          assigneeAgentId: string | null;
+          sessionJson: string;
+          sessionDisplayId: string | null;
+          updatedAt: string;
+          connectorId: string | null;
+        }>(
+          `SELECT t.id AS id, t.company_id AS companyId, t.assignee_agent_id AS assigneeAgentId,
+                  t.session_json AS sessionJson, t.session_display_id AS sessionDisplayId,
+                  t.updated_at AS updatedAt, a.connector_id AS connectorId
+             FROM tasks t
+             LEFT JOIN agents a ON a.id = t.assignee_agent_id
+            WHERE t.session_json IS NOT NULL
+              AND t.id NOT IN (SELECT task_id FROM task_sessions)`,
+        );
+        for (const task of legacyTasks) {
+          const sessionId = randomUUID();
+          db.prepare(
+            `INSERT OR IGNORE INTO task_sessions (id, company_id, task_id, agent_id, adapter_type, session_params_json, session_display_id, created_at, updated_at)
+             VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+          ).run(
+            sessionId,
+            task.companyId,
+            task.id,
+            task.assigneeAgentId ?? "unassigned",
+            task.connectorId ?? "unknown",
+            task.sessionJson,
+            task.sessionDisplayId,
+            task.updatedAt,
+            task.updatedAt,
+          );
+          db.prepare(
+            "UPDATE tasks SET active_session_id = ? WHERE id = ? AND active_session_id IS NULL",
+          ).run(sessionId, task.id);
+        }
+      },
+    },
   ];
 
   for (const migration of migrations) {

src/main/database/tasks-crud.ts

@@ -185,36 +185,74 @@ export function buildTaskCrudMethods(
       sessionJson: Record<string, unknown> | null,
       sessionDisplayId: string | null,
     ): void {
+      const ts = nowIso();
+      const sessionParamsJson = sessionJson ? JSON.stringify(sessionJson) : null;
+      const taskRow = getDb()
+        .prepare(
+          "select company_id as companyId, assignee_agent_id as assigneeAgentId, active_session_id as activeSessionId from tasks where id = ?",
+        )
+        .get(taskId) as
+        | { companyId: string; assigneeAgentId: string | null; activeSessionId: string | null }
+        | undefined;
+      if (!taskRow) throw new Error(`Task not found: ${taskId}`);
+      if (taskRow.activeSessionId) {
+        getDb()
+          .prepare(
+            "update task_sessions set session_params_json = ?, session_display_id = ?, updated_at = ? where id = ?",
+          )
+          .run(sessionParamsJson, sessionDisplayId, ts, taskRow.activeSessionId);
+        return;
+      }
+      if (sessionParamsJson === null && sessionDisplayId === null) return;
+      const agentId = taskRow.assigneeAgentId ?? "unassigned";
+      const agent = taskRow.assigneeAgentId
+        ? (getDb()
+            .prepare("select connector_id as connectorId from agents where id = ?")
+            .get(taskRow.assigneeAgentId) as { connectorId: string } | undefined)
+        : undefined;
+      const adapterType = agent?.connectorId ?? "unknown";
+      const sessionId = randomUUID();
       getDb()
         .prepare(
-          "update tasks set session_json = ?, session_display_id = ?, updated_at = ? where id = ?",
+          `insert into task_sessions (id, company_id, task_id, agent_id, adapter_type, session_params_json, session_display_id, created_at, updated_at)
+           values (?, ?, ?, ?, ?, ?, ?, ?, ?)`,
         )
         .run(
-          sessionJson ? JSON.stringify(sessionJson) : null,
+          sessionId,
-          sessionDisplayId,
+          taskRow.companyId,
-          nowIso(),
           taskId,
+          agentId,
+          adapterType,
+          sessionParamsJson,
+          sessionDisplayId,
+          ts,
+          ts,
         );
+      getDb()
+        .prepare("update tasks set active_session_id = ?, updated_at = ? where id = ?")
+        .run(sessionId, ts, taskId);
     },
 
     getTaskSession(taskId: string): {
       sessionParams: Record<string, unknown> | null;
       sessionDisplayId: string | null;
     } {
+      const taskRow = getDb()
+        .prepare("select active_session_id as activeSessionId from tasks where id = ?")
+        .get(taskId) as { activeSessionId: string | null } | undefined;
+      if (!taskRow?.activeSessionId) {
+        return { sessionParams: null, sessionDisplayId: null };
+      }
       const row = getDb()
         .prepare(
-          "select session_json as sessionJson, session_display_id as sessionDisplayId from tasks where id = ?",
+          "select session_params_json as sessionParamsJson, session_display_id as sessionDisplayId from task_sessions where id = ?",
         )
-        .get(taskId) as
+        .get(taskRow.activeSessionId) as
-        | { sessionJson: string | null; sessionDisplayId: string | null }
+        | { sessionParamsJson: string | null; sessionDisplayId: string | null }
         | undefined;
       let sessionParams: Record<string, unknown> | null = null;
-      if (row?.sessionJson) {
+      if (row?.sessionParamsJson) {
-        try {
+        sessionParams = JSON.parse(row.sessionParamsJson) as Record<string, unknown>;
-          sessionParams = JSON.parse(row.sessionJson) as Record<string, unknown>;
-        } catch {
-          sessionParams = null;
-        }
       }
       return {
         sessionParams,