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/fixture/agent-plugin.ts

@@ -0,0 +1,12 @@
+// Every export in this file must be a plugin function — `getLegacyPlugins`
+// (src/plugin/index.ts) throws on anything else. Test constants live in
+// `agent-plugin.constants.ts`.
+export default async () => ({
+  config: async (cfg: { agent?: Record<string, unknown> }) => {
+    cfg.agent = cfg.agent ?? {}
+    cfg.agent["plugin_added"] = {
+      description: "Added by a plugin via the config hook",
+      mode: "subagent",
+    }
+  },
+})