[task] Improve API key setup guidance with decision tree and clearer instructions

[github-actions]

Objective

Make the API key setup process clearer by adding a decision tree to help users choose the right API and providing better context for GitHub PAT creation.

Context

Current API setup assumes too much knowledge:

• Users don't know which API to choose
• GitHub PAT creation steps assume familiarity with concepts
• Error messages are unclear

Related to #4257 (Documentation Noob Test Report)

Approach

1. Add a decision tree for choosing API providers
2. Clarify GitHub PAT creation with explanations of concepts
3. Improve error message guidance

Files to Modify

• docs/src/content/docs/setup/quick-start.mdx

Implementation Details

Add Decision Tree Before Step 3

### Which API Should I Use?

Choose based on what you already have:

- **✅ I have GitHub Copilot** → Use GitHub Copilot CLI (Step 3a)
  - Pros: Free with subscription, well-integrated
  - Cons: Requires Copilot subscription

- **✅ I want pay-as-you-go** → Use Claude or OpenAI (Step 3b/3c)
  - Pros: No subscription needed, predictable costs
  - Cons: Small per-workflow costs (~$1-15)
  
- **❓ I'm not sure** → Try GitHub Copilot CLI first if you have a subscription, otherwise use Claude API (most popular)

Improve GitHub PAT Instructions

Add explanatory context:

#### Understanding GitHub Personal Access Tokens

**What is a PAT?** A Personal Access Token is like a password that gives gh-aw permission to access your repositories.

**Why personal account?** Organizational repositories can use the token, but the token itself must be created under your personal account (not the organization's settings).

**Fine-grained vs Classic?** 
- **Fine-grained tokens** (recommended): More secure, can limit to specific repositories
- **Classic tokens**: Broader access, easier to set up

**Token permissions needed:**
- `contents: read` - To read repository files
- `actions: write` - To trigger workflows

Clarify Error Messages

Replace:

If you are not finding this option, review steps 2 and 3 above.

With:

**Can't find "Fine-grained tokens"?**
1. Ensure you're in your personal account settings (not organization)
2. Look under "Developer settings" → "Personal access tokens"
3. Try the beta "Fine-grained tokens" option
4. If unavailable, use "Tokens (classic)" instead

Acceptance Criteria

• Decision tree helps users choose API provider
• GitHub PAT concepts are explained (what, why, types)
• Error guidance is specific and actionable
• All three API options have equal clarity
• Documentation builds without errors

Priority

MEDIUM - Identified as medium-term improvement (half-day effort)

AI generated by Plan Command for discussion #4257