feat(studio): redesign agent builder workspace

[evanlowe]

Summary

• rebuild the Studio agent creation workspace around the Figma canvas, navigation, chat drawer, and configuration panel
• add nested agent canvas presentation, responsive panel motion, and Figma-aligned single/two-column debug comparison flows
• connect multi-turn agent-building conversations to a standard completion-backed service with explicit generate_agent tool invocation
• reuse Apps SDK UI form controls, persist safe conversation state locally, and update bundled Studio assets

Verification

• cd frontend && npm test — 751 tests passed
• cd frontend && npm run build — passed
• uv run pytest -q tests/cli/test_generated_agent_conversation.py — 7 tests passed
• uvx pre-commit run --all-files — Ruff, formatting, and Gitleaks passed
• uv run --with pyright pyright --pythonpath .venv/bin/python veadk/cli/generated_agent_conversation.py tests/cli/test_generated_agent_conversation.py — 0 errors
• browser geometry check — single debug group 471px; two groups 942px with 471px columns

Notes

• full-file Pyright for the existing cli_frontend.py still reports 18 baseline diagnostics outside the newly added conversation routes