docs(readme): architecture diagram for 'Design, in one paragraph'

[nickroci]

Summary

Adds a Mermaid flowchart at the top of the Design, in one paragraph section so first-time readers get the conceptual mental model before they hit the bio references and numbered principles.

```
[Sessions A · B · C] ──events──▶ Ultan ──primes next turn──▶ back to sessions

Inside Ultan:
Librarian (Sonnet) ──proposes──▶ Scholar (Opus, the gate) ──writes approved──▶ Knowledge library
Knowledge library ──surfaces──▶ Librarian
```

Multiple agent sessions feed Ultan with conversation events; the Librarian watches and proposes what's worth remembering; the Scholar is the gate (approves or vetoes each proposal); approved proposals land in the markdown library; the library then surfaces relevant entries back to the Librarian (for context) and primes future sessions (Tier-1 ambient).

Deliberately simpler than the detailed How it works mermaid further down — that one carries the full event-log → tailer → debounce → librarian pool → scholar → reconciler chain. This one is the conceptual agents-↔-Ultan loop the design paragraph is about.

Test plan

• Renders on GitHub (Mermaid block is standard `flowchart LR` syntax)
• Placed inside the section it describes; doesn't disturb the existing structure (just inserts between the intro sentence and the numbered list)

🤖 Generated with Claude Code