Epic: Assistant workflow & plan completion

[ApocDev]

Finish the assistant as an active planning surface: plans should include their identified follow-on work, and the same contextual conversation should be reachable throughout the app.

Sub-issues:

• Assistant: recurse building-supply into component blocks and emit material scale-ups #132 — recurse building supply into component blocks and emit material scale-ups.
• Assistant panel: talk to the assistant from any page, with page context #131 — open the assistant from any page with focused page context.

Order: make generated plans complete first, then broaden access to the assistant.