chore(agent): upgrade agent default model to gemini-3.6-flash

[dembrane-sam-bot]

What is this change?

• Upgrades the default model for the echo/agent (the dembrane chat agent) from gemini-3.5-flash to gemini-3.6-flash.
• Updates corresponding documentation, design diagrams, and mock tests to reference the new version.

What led to this?

• Jor requested an investigation into upgrading Sam and the Dembrane Chat agent to Gemini 3.6 Flash (released July 21, 2026).
• Gemini 3.6 Flash brings significant optimizations for agentic workflows, better code generation, and is more cost-effective.

Confidence?

• High. Code changes are simple default-value and test-mock updates.