Add --yes flag to skills add command in README

[ayanmajumdar-m]

This PR updates the installation command in the README by adding the --yes flag to the npx skills add command.
What Changed
Before:
npx skills add flutter/skills --skill '' --agent universal
After:
npx skills add flutter/skills --skill '' --agent universal --yes
Why
The --yes flag enables non-interactive execution by automatically confirming prompts during installation. This improves automation support and provides a smoother experience for users running the command in CI/CD pipelines, scripts, or headless environments.
Fixes

Fixes issue where the installation command pauses for manual confirmation during automated execution.

Pre-launch Checklist

I read the Contributor Guide and followed the process outlined there for submitting PRs.

I read the Tree Hygiene wiki page, which explains my responsibilities.

I read the Flutter Style Guide recently, and have followed its advice.

I signed the CLA.

I listed at least one issue that this PR fixes in the description above.

I updated/added relevant documentation (doc comments with ///).

This PR is test-exempt (documentation-only change).

All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request updates the README.md to include the --yes flag in the example command for adding skills. A review comment suggests further improving the command for CI/CD environments by adding the -y flag to the npx call to prevent interactive prompts during package installation.