MR-6 — Subagents read from `AGENTS.xml`, not from inferred patterns

If an orchestrating Claude session needs to delegate but the right subagent isn’t in AGENTS.xml, the correct response is add it to the XML, not improvise a delegation.

Improvised delegation is how the delegation grammar rots. Six weeks later nobody can reproduce who handled what.

Adding a subagent to AGENTS.xml is a small commit; it’s fine, do it.