docs(skills): move + update Custom Subagents (Guides → Skills & Agents) [carries the IA move]

[Aaronontheweb]

Part of #95 · Group 3 · MOVE guides/custom-subagents.md → skills/custom-subagents.md + UPDATE · self-serve

This issue carries the approved IA restructure:

• Rename the sidebar section Skills → Skills & Agents in astro.config.mjs and add the section's pages.
• Move the file guides/custom-subagents.md → skills/custom-subagents.md.
• Append the 301 to public/_redirects: /guides/custom-subagents/ /skills/custom-subagents/ 301 (same pattern as the existing /guides/connecting-slack/ move).
• Update every internal link to the old slug across the docs.

Content update: add a 'Distributing across your org' section (or prominent cross-link to #6); document the agents/.server-feeds/ managed dir and local > managed-feed precedence; stop framing subagents as local-only.

Deps: #6.