📝 Rename test-loop "blue" phase to "type"; cite TDD

The type-first spec step was labelled "Blue", which collides with the
Red-Green-Blue convention where blue means the refactor phase (and our
loop already has an explicit Refactor step). Rename phase 1 to Type and
describe the loop as type -> red -> green -> refactor.

Tie the discipline to its established name: Type-Driven Development
(Edwin Brady), quoting the Idris framing -- treat the type as the plan,
let the compiler/type-checker drive you to a program that satisfies it.
Section heading becomes "Testing discipline (type-driven)"; update the
AGENTS.md pointer + anchor. Add "idris" to the cspell dictionary.

De-duplicate: CONTRIBUTING.md no longer re-lists the banned directives
(@ts-ignore, as casts, ...); it links to AGENTS.md "Never do", the
single home for that rule, so the lists can't drift.

cspell.json

@@ -28,7 +28,8 @@
         "Zilla",
         "kacl",
         "bestikk",
-        "silverwind"
+        "silverwind",
+        "idris"
     ],
     "ignorePaths": ["dist", "node_modules", "coverage", "*.svg", ".gitignore"]
 }