feat(test): split env config — .env (prod) vs .env.sandbox (sandbox)

provisionSandbox.ts now loads its own .env.sandbox (via @std/dotenv loadSync)
instead of the shared .env, so prod (main.ts → .env) and sandbox
(provisionSandbox.ts → .env.sandbox) configs don't collide. .gitignore widened
to .env* (keeping .env.example tracked). .env.example rewritten to document the
two-file convention + the per-env kubectl secret sources, including the caveat
that a prod-seeded sandbox uses PROD's admin password.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/.gitignore

@@ -1,5 +1,6 @@
-# Secrets — never commit. The root .gitignore already covers .env and *.key;
-# this is defense-in-depth for the provisioning POC.
-.env
+# Secrets — never commit. Covers .env (prod, main.ts) and .env.sandbox
+# (sandbox, provisionSandbox.ts), plus any generated *.key.
+.env*
+!.env.example
 .ai_agent_sandbox.key
 *.key