Route task decomposition through the plan tracker

[Luhaozhu]

Summary

Follow-up to #29. The default system prompt's task-decomposition rule predated the update_plan plan tracker and still forced the model to write a text checklist into the reply body — so complex tasks could end up with both a body checklist and the plan bar showing the same list.

• System prompt workflow §3 now routes decomposition through update_plan when the tool is available: the checklist lives in the plan bar above the chat input and the reply body no longer repeats it. Contexts without the tool (automations, channels, sub-agents) keep the original in-reply checklist behavior.
• Pure-text compound questions (e.g. "summarize A and B, then compare") no longer force a checklist; instead the answer must cover every requested item in its own section.
• The update_plan prompt section gains the matching "don't repeat the checklist in the body" instruction, closing the loop from both sides.
• The deliverable-completeness rule — every requested item must have its own section in the produced document/slides/spreadsheet — is unchanged.

Testing

• Multi-deliverable task (Excel + chart): plan bar checklist, no duplicated text checklist in the reply.
• Pure-text compare/summarize question: no plan bar, answer covers all requested items in sections.
• Manual plan mode still generates a trailing verify-and-fix step.