[d2] add tooltips to the L1 diagram nodes

[rohitsux]

The L1 system context diagram shows actors and the Carryover box but doesn't carry hover-text. d2 supports tooltip: on any node. Add tooltips to:

• the AI Agent actor (list the six supported tools)
• the Carryover box (one-line summary)
• the AGENTS.md / CLAUDE.md output node (mention the privacy split)

Touch architecture/d2/l1_sketch.d2. Re-render with d2 --sketch architecture/d2/l1_sketch.d2 architecture/d2/l1_sketch.svg. Commit both.