docs: clarify Claude bootstrap flow

[HarveySang]

Summary

• Clarify that Claude Code or Codex should be opened from the OpenGUI repo root and asked to read the bootstrap skill.
• Explain that Claude Code / Codex does not connect to the phone directly; it guides the local backend, Android client, and adb setup flow.
• Add the same bootstrap explanation and smoke test to docs/get-started.md.

Test Plan

• git diff --check

Refs #33, #34