[Docs]: User guide with an annotated example session and FAQ

[shauryagangrade]

Problem

README lists commands but there's no walkthrough: a user can't see what a
real session looks like, what /models #n selection means, how --yes
changes behavior, or answers to common questions (rate limits, tool-support
404s, Ollama setup).

Proposed approach

• Add docs/user-guide.md with an annotated example session (banner,
  prompt, a streamed answer, a gated bash call, model switch) and a FAQ
  (429s, "No endpoints found that support tool use", --yes safety,
  history location, uninstalling).
• Link from the README under "Use".

Where to look

• README.md:40-51 (commands) for the anchor content.
• Real behavior: gcode/ui.py markup, gcode/cli.py commands.

Acceptance criteria

• A reader who has never run GCode can follow the example end-to-end.
• FAQ answers match current behavior.

Difficulty

Low — good first issue (docs).