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/product.md

@@ -0,0 +1,18 @@
+# Product Overview
+
+[Brief description of what this product does and who it serves]
+
+## Core Capabilities
+
+[3-5 key capabilities, not exhaustive features]
+
+## Target Use Cases
+
+[Primary scenarios this product addresses]
+
+## Value Proposition
+
+[What makes this product unique or valuable]
+
+---
+_Focus on patterns and purpose, not exhaustive feature lists_