feat(cli): add planoai launch group + claude-desktop integration

[Spherrrical]

Summary

• New planoai launch command group: claude-cli, codex, and claude-desktop (replaces the old planoai cli-agent ...).
• planoai launch claude-desktop ports Ollama's cmd/launch/claude_desktop.go flow 1:1 — flips Claude Desktop into 3p gateway mode pointed at the local Plano gateway, with .bak backups and --restore. --config <path> auto-runs planoai up first if Plano isn't already running.
• New demo: demos/llm_routing/frontier_model_routing/ (DigitalOcean Sonnet 4.6 + GPT 5.5 + Opus 4.7) using v0.4.0 top-level routing_preferences with ordered fallbacks.

Test plan

• cd cli && uv run pytest -v passes (currently 130 ✅)
• On macOS: planoai launch claude-desktop --config demos/llm_routing/frontier_model_routing/config.yaml flips Claude Desktop into 3p mode, models from /v1/models show up in the in-app picker
• planoai launch claude-desktop --restore reverts cleanly
• planoai launch claude-cli and planoai launch codex still work
• Run on a second machine to confirm cross-machine reproducibility