Install instructions should remind user to set COPILOT_GITHUB_TOKEN

[mrjf]

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:

**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 "<your-token>"`

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.