Replace AGENTS.md with simplified CLAUDE.md file (#5604)

I've been wanting to replace AGENTS.md with something simpler. And since
we're already using `.claude/skills` we might as well adopt their
structure for this too. I wish there were a universal standard for this
stuff.

Author: jtraglia

.claude/CLAUDE.md

@@ -0,0 +1,12 @@
+## Structure
+
+- Specs are markdown; executable Python is generated from them.
+- Each spec states only what it changes; the rest is inherited.
+- Changes to old specs may require matching updates to newer ones.
+- Presets are comptime; configs are runtime.
+
+## Constraints
+
+- Do not edit generated Python spec files.
+- Do not assume how the specs behave, as they are evolving.
+- Do not change stable spec behavior without explicit permission.