chore: add kiro spec-driven tooling and steering docs

Add the Kiro-style agentic SDLC tooling used to drive this project: the
.opencode/skills/kiro-* skill set (discovery, spec init/requirements/design/
tasks, validation, implementation, review, steering), the .kiro/settings
templates, and AGENTS.md describing the workflow. Update the spec docs
(assessment/design/requirements/tasks) to current state.

Scope tests/test_opencode_skills.py to the project's own openultrasast-*
skills so the third-party kiro-* tooling (which uses a different frontmatter
convention) no longer breaks the frontmatter check.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

.kiro/settings/templates/specs/init.json

@@ -0,0 +1,24 @@
+{
+  "feature_name": "{{FEATURE_NAME}}",
+  "created_at": "{{TIMESTAMP}}",
+  "updated_at": "{{TIMESTAMP}}",
+  "language": "en",
+  "phase": "initialized",
+  "approvals": {
+    "requirements": {
+      "generated": false,
+      "approved": false
+    },
+    "design": {
+      "generated": false,
+      "approved": false
+    },
+    "tasks": {
+      "generated": false,
+      "approved": false
+    }
+  },
+  "ready_for_implementation": false
+}
+
+