Install instructions should remind user to set COPILOT_GITHUB_TOKEN

## Problem

After following `install.md`, the autoloop workflow fails on first run because `COPILOT_GITHUB_TOKEN` is not set. The error is not obvious:

```
Error: None of the following secrets are set: COPILOT_GITHUB_TOKEN
```

## Suggestion

Add a short reminder at the end of Step 5 (after the PR is created) in `install.md`, something like:

```markdown
**Important:** Before the workflow can run, add a `COPILOT_GITHUB_TOKEN` secret to your repository:

1. [Create a fine-grained PAT](https://github.com/settings/personal-access-tokens/new?name=COPILOT_GITHUB_TOKEN&description=GitHub+Agentic+Workflows+-+Copilot+engine+authentication&user_copilot_requests=read) with **Account permissions > Copilot Requests > Read**.
2. Add it to your repo: `gh aw secrets set COPILOT_GITHUB_TOKEN --value ""`
```

Also have the installing agent print this after completing the install steps, so the user sees it immediately rather than discovering it from a failed Actions run.