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/steering/structure.md

@@ -0,0 +1,41 @@
+# Project Structure
+
+## Organization Philosophy
+
+[Describe approach: feature-first, layered, domain-driven, etc.]
+
+## Directory Patterns
+
+### [Pattern Name]
+**Location**: `/path/`  
+**Purpose**: [What belongs here]  
+**Example**: [Brief example]
+
+### [Pattern Name]
+**Location**: `/path/`  
+**Purpose**: [What belongs here]  
+**Example**: [Brief example]
+
+## Naming Conventions
+
+- **Files**: [Pattern, e.g., PascalCase, kebab-case]
+- **Components**: [Pattern]
+- **Functions**: [Pattern]
+
+## Import Organization
+
+```typescript
+// Example import patterns
+import { Something } from '@/path'  // Absolute
+import { Local } from './local'     // Relative
+```
+
+**Path Aliases**:
+- `@/`: [Maps to]
+
+## Code Organization Principles
+
+[Key architectural patterns and dependency rules]
+
+---
+_Document patterns, not file trees. New files following patterns shouldn't require updates_