Refactor: Extract Custom Agent Files section to dedicated reference page

[Copilot]

Addresses review feedback from #2851 to move the Custom Agent Files section from engines.md into its own reference page for better documentation organization.

Changes

• Created docs/src/content/docs/reference/custom-agents.md

  • Extracted complete Custom Agent Files documentation (creation, usage, requirements, migration)
  • Configured sidebar order at 650 (between AI Engines and Tools)

• Updated docs/src/content/docs/reference/engines.md

  • Replaced full section with brief summary and cross-reference link
  • Maintains content accessibility while reducing file size

Result

The Custom Agent Files feature now has dedicated reference documentation with proper navigation:

Merges into: #2851

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• telemetry.astro.build
  • Triggering command: node /home/REDACTED/work/gh-aw/gh-aw/docs/node_modules/.bin/astro build (dns block)
  • Triggering command: node /home/REDACTED/work/gh-aw/gh-aw/docs/node_modules/.bin/astro preview --port 4321 (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)