Adding a staging skill

[sfshaza2]

Description of what this PR is changing or adding, and why:

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[gemini-code-assist]

Code Review

The pull request introduces a new Markdown document, .agents/skills/stage-site/SKILL.md, which details a four-step process for locally staging the Flutter site. The review comments suggest improving the readability of this new document by wrapping two lines that exceed the 80-character limit, as per the project's style guide.

[gemini-code-assist]

This line exceeds the 80-character limit specified in the PR's presubmit checklist for semantic line breaks. Please wrap the line to adhere to the style guide and improve readability.

Suggested change

	Ensure that any changed code excerpts are properly run and synced to the Markdown files:
	Ensure that any changed code excerpts are properly run
	and synced to the Markdown files:

[gemini-code-assist]

This line exceeds the 80-character limit specified in the PR's presubmit checklist for semantic line breaks. Please wrap the line to adhere to the style guide and improve readability.

Suggested change

	Finally, serve a local development server of the site so the user can preview the changes:
	Finally, serve a local development server of the site
	so the user can preview the changes: