Adopt the glossary's vocabulary in the mutable design docs

"control plane" -> controller and "substrate" -> foundation throughout
03-DESIGN, 00-META and the README, with 06-the-control-plane.md and
07-the-substrate.md renamed to 06-the-controller.md and 07-the-foundation.md.
The immutable 02-DECISIONS records keep their original wording (and links to
them are unchanged) — a term retired here may still appear there, which the
glossary explains how to read.

Claude-Session: https://claude.ai/code/session_01D6qtiYU3P9jk3pnAXyAFyx

00-META/context.md

@@ -30,7 +30,7 @@ named, and nothing should be designed around a particular one existing.
 - **A hosted model provider** supplies the thinking for non-human agents, drawn from a
   shared pool of subscriptions — which is why budget pacing is a first-class concern.
 - **Long-lived user services** rather than an orchestrator. No cluster scheduler, no cloud
-  control plane.
+  controller.
 
 Defaults, not mandates. A second model provider is anticipated by design; nothing in the
 domain may assume one vendor's credential lifecycle.