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/types.ts

@@ -38,6 +38,7 @@ export interface ScheduledTask {
   prompt: string;
   schedule_type: 'cron' | 'interval' | 'once';
   schedule_value: string;
+  context_mode: 'group' | 'isolated';
   next_run: string | null;
   last_run: string | null;
   last_result: string | null;