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
- System prompt injection for routing
- V1 requirements: C4 docs, ADR, AGENTS.md, debug-log.md
- Design documents in docs/

packages/opencode/test/fixture/agent-plugin.constants.ts

@@ -0,0 +1,6 @@
+// Separate file because every export in `agent-plugin.ts` must be a function.
+export const PLUGIN_AGENT = {
+  name: "plugin_added",
+  description: "Added by a plugin via the config hook",
+  mode: "subagent",
+} as const