docs: user guide (GUIDE.md) — first-run walkthrough for new users

[Hmbown]

Source

Flash branch:

• 223fdff3f docs: add user-friendly CodeWhale guide (GUIDE.md)
• 81e46af2c docs: expand user guide (GUIDE.md) and link from README docs table

What

A friendly GUIDE.md for first-time users: what CodeWhale is, how to get an API key, first-run walkthrough, mode switching, common workflows. Linked from README. Complements the reference docs in docs/.

Scope

Documentation only. Fits v0.8.48's first-run and defaults milestone.

[Hmbown]

Investigated this for v0.8.48. Two flash-branch drafts already exist (commits 223fdff and 81e46af, the latter at ~374 lines with 10 sections + FAQ).

Proposed outline for docs/GUIDE.md (~400-500 lines):

1. Welcome to CodeWhale — what is this, what can I do
2. First Launch — install, configure API key, verify it works
3. Your First Task — explain a codebase, ask a question
4. Understanding the Interface — header, transcript, composer, sidebar, footer
5. Modes — Agent, Yolo, Plan — when to use each
6. Slash Commands — /model, /provider, /config, /compact, /review
7. Working with Tools — approvals, sandbox, file operations
8. Sub-agents and Parallel Work — fanout, background tasks
9. Skills — installing and using community skills
10. Getting Help — /doctor, logs, GitHub issues, community
11. FAQ — common issues and solutions

Each section should be ~30-50 lines, tutorial-style (not reference), with concrete examples. End each with a Next link to the relevant reference doc.