feat(v2): add update_task MCP tool, dedup list_tasks by series

update_task lets the agent adjust prompt/recurrence/processAfter/script
on a live scheduled task without losing the series id the user already
knows. Empty string clears recurrence/script.

list_tasks now groups by series_id so recurring tasks show as one row
(the live pending/paused occurrence) instead of one per firing — the
id displayed is the stable series handle that update/cancel/pause/resume
all match against.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

groups/main/CLAUDE.md

@@ -276,6 +276,8 @@ The task will run in that group's context with access to their files and memory.
 
 For any recurring task, use `schedule_task`. Frequent agent invocations — especially multiple times a day — consume API credits and can risk account restrictions. If a simple check can determine whether action is needed, add a `script` — it runs first, and the agent is only called when the check passes. This keeps invocations to a minimum.
 
+Use `list_tasks` to see existing tasks (one row per series with the stable id), and `update_task` / `cancel_task` / `pause_task` / `resume_task` to modify them. Prefer `update_task` over cancel + reschedule when adjusting an existing task.
+
 ### How it works
 
 1. You provide a bash `script` alongside the `prompt` when scheduling