feat(tasks): add <log> append zone and background-mode description gate

[saimanoj]

Recurring tasks were overwriting their description each fire because the agent kept rewriting / zones (replace semantics) and any update bundling a scheduling field with description still wholesale- replaced the page via setPageContentFromHtml.

• Add as a third structured zone with APPEND semantics, alongside / (replace). Server parser, client editor node, and the zone merger all updated.
• Add clearLog flag on update_task to wipe only — for the accumulate-daily, send-weekly, clear pattern.
• Gate / writes behind isBackgroundExecution: scheduled fires and background runs may only append to (or clearLog). Live chat unchanged. The plan is frozen unless the user is in the loop.
• Route updateScheduledTask description writes through upsertPageSection instead of setPageContentFromHtml so scheduling-bundled updates can't obliterate .
• Update capabilities and trigger_context prompts with the new zone model and background restriction.
• Add 28 unit tests covering the merger (replace vs append, validation, user-prose preservation) and the background gate (rejection paths, allowed paths, scheduling+description regression).