Add context_mode option for scheduled tasks

Scheduled tasks can now run in either:
- "group" mode: uses the group's conversation session for context
- "isolated" mode: runs with a fresh session (previous behavior)

The tool description guides the agent on when to use each mode and
prompts them to ask the user if unclear. Group mode is now the default.

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

src/db.ts

@@ -64,6 +64,11 @@ export function initDatabase(): void {
   try {
     db.exec(`ALTER TABLE messages ADD COLUMN sender_name TEXT`);
   } catch { /* column already exists */ }
+
+  // Add context_mode column if it doesn't exist (migration for existing DBs)
+  try {
+    db.exec(`ALTER TABLE scheduled_tasks ADD COLUMN context_mode TEXT DEFAULT 'isolated'`);
+  } catch { /* column already exists */ }
 }
 
 /**
@@ -131,8 +136,8 @@ export function getMessagesSince(chatJid: string, sinceTimestamp: string): NewMe
 
 export function createTask(task: Omit<ScheduledTask, 'last_run' | 'last_result'>): void {
   db.prepare(`
-    INSERT INTO scheduled_tasks (id, group_folder, chat_jid, prompt, schedule_type, schedule_value, next_run, status, created_at)
-    VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)
+    INSERT INTO scheduled_tasks (id, group_folder, chat_jid, prompt, schedule_type, schedule_value, context_mode, next_run, status, created_at)
+    VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
   `).run(
     task.id,
     task.group_folder,
@@ -140,6 +145,7 @@ export function createTask(task: Omit<ScheduledTask, 'last_run' | 'last_result'>
     task.prompt,
     task.schedule_type,
     task.schedule_value,
+    task.context_mode || 'isolated',
     task.next_run,
     task.status,
     task.created_at