feat: add supervisor CLAUDE.md, tdd skill prompt, and env example

config/supervisor/tdd.md

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+# TDD Skill
+
+## Iron Law
+
+NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST.
+
+## Red phase
+
+- Write exactly one test. One behavior. Name must describe the behavior clearly.
+- Run the test suite. Confirm the test FAILS.
+- If the test passes immediately: it tests existing behavior or is vacuous.
+  Return status "fail" with message explaining why the test is wrong.
+- Do not write any implementation code in this phase.
+
+## Green phase
+
+- Write the minimal code to make the failing test pass. Nothing more.
+- YAGNI: no extra parameters, no future-proofing, no clever abstractions.
+- Run the test suite. Confirm it PASSES.
+- If tests fail: fix the implementation, not the test. Max 3 attempts.
+
+## Refactor phase
+
+- Improve structure, naming, or clarity only. No new behavior.
+- Tests must remain green after every change.
+- If tests break during refactor: revert that change, return status "fail".