docs: add ARCHITECTURE.md — technical decision rationale for engineers

[CodeNinjaSarthak]

Summary

• Adds ARCHITECTURE.md at the repo root: six sections covering orchestrator branching ownership, session state design, refinement as a distributed-systems problem, cost tracking design, deliberate omissions, and known test suite warnings
• Three Mermaid diagrams: session lifecycle state machine, refinement execution flow, cost tracking flow — each placed before the prose that references it
• Satisfies the architecture diagram deliverable from README final pass: demo GIF, architecture diagram, what-I-didn't-build section #72

What this covers from #72

The "architecture diagram" and "what-I-didn't-build section" items are covered here. The demo GIF and any remaining README prose from #72 can land separately since this document stands alone.

Test plan

• Render ARCHITECTURE.md in GitHub preview — confirm all three Mermaid diagrams render without errors
• Run python scripts/check_mermaid.py ARCHITECTURE.md locally to validate diagram syntax
• Grep for parentheses in Mermaid blocks: awk '/\``mermaid/,/```/' ARCHITECTURE.md | grep '('` — should return no matches

Closes #72