docs: add SDD specs + user requirements + tests

- .agents/specs/: spec-driven-dev docs for arcrun MVP, auth-recipe,
  credential-primitives-wasm (active refactor), landing-page,
  sdk-and-website, u6u-core-mvp, u6u-platform-evolution.
- .agents/steerings/tech.md: detailed tech stack rationale.
- docs/user_requirements/: long-form requirements incl. credential
  primitives, pages spec, py strategy analysis.
- tests/: end-to-end harness scaffolding.

These are the durable context backing CLAUDE.md's SDD protocol.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/arcrun-test/hello.yaml

@@ -0,0 +1,13 @@
+# arcrun hello world workflow
+# 執行：acr run hello --input input="Hello, arcrun!"
+
+name: hello
+description: "Hello world — 示範字串轉大寫"
+
+flow:
+  - "input >> ON_SUCCESS >> transform"
+
+config:
+  transform:
+    component: string_ops
+    operation: upper