Update agent docs for background dev servers

[matthewp]

Changes

• Replaces the old bgproc guidance in AGENTS.md with Astro's native background dev server commands.
• Updates repo-local agent skills to use astro dev --background, astro dev stop, astro dev status, and astro dev logs for dev server lifecycle management.

Testing

• No test code changed; this is a documentation-only update.

Docs

• Updates internal agent guidance and local skill docs directly.

[changeset-bot]

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Latest commit: 6fc7865

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