Draft: 7.1 Document AI Catalog agents as sub-agents in custom flows

Parked for 19.6. Everything on these pages describes the include section, and in 19.5 a flow with an include section validates its agents but does not run them. The run-time path ships in 19.6 on a pull model, and these docs merge with it. This MR is now tracked by #633101 under the execution epic &24006. The save-time issues it was written for (#628153 (closed), #628317 (closed)) are closed as complete.

What does this MR do and why?

Documents AI Catalog agents as sub-agents in custom flows (#628153 (closed)), and the soft deletion of a referenced agent (#628317 (closed)). The save-time code is in !258326 (merged) and !259407 (merged), behind the ai_catalog_flow_agent_references feature flag. Neither MR changed the docs.

Page	Change
flows/custom_flows_schema.md	New include section: the YAML shape, the exact-version rule, the limits, the visibility rules, and what happens at save time.
flows/custom.md	Removes the note that a flow cannot call a custom agent. Adds a history entry. "Delete a flow" no longer says the delete is permanent.
flows/claude-edit-v1-flow-registry.md	The subagents row also accepts include entries.
agents/custom.md	New "In a custom flow" section under "Use an agent". "Delete an agent" no longer says the delete is permanent.
ai_catalog.md	The visibility rules also apply to referenced agents. New "Delete an item" section with the one soft-delete rule.

Before this merges: add the run-time behaviour under the pull contract, and move the history entry to the milestone the run-time path ships in.

References

Closes #633101

• Parent epic: &24006 (execution). Save-time context: #628153 (closed), #628317 (closed) under &23526 (closed)
• Code: !258326 (merged), !259407 (merged)

How to set up and validate locally

scripts/lint-doc.sh