Incognito and sources follow-ups: delete incognito outputs, sources for task runs #291

Follow-ups to #286 (incognito chats) and #290 (sources under answers).

• Incognito leftovers: when an incognito chat is deleted, also delete the long tool results it saved under files/outputs/, because those are incidental, not something the user asked for. Tasks it created and files it wrote on request stay.
• Sources for task runs: record the same sources (pages, emails, files, apps, memories) for each task run, and show the chips in the task's run history.

Done when: both are done and checked in a real run.