feat(aircoding): AirCoding V2 baseline — deterministic multi-agent architecture

Forked from OpenCode v1.17.4 with multi-agent system:
- 5 agents: aircoding, scheduler, worker, architect, reviewer
- Deterministic DAG scheduling engine (coordinator_tick)
- Tool whitelists as hard enforcement
- AirCoding validation plugin
- V1 requirements: C4 docs, ADR, AGENTS.md, debug-log.md
- Design documents in docs/

packages/opencode/test/fake/skill.ts

@@ -0,0 +1,8 @@
+import { Effect, Layer } from "effect"
+import { Skill } from "../../src/skill"
+
+export const empty = Layer.mock(Skill.Service)({
+  dirs: () => Effect.succeed([]),
+})
+
+export * as SkillTest from "./skill"