feat(opencode): cut plugin system and remote behavior feeds; add SessionContext assembly and Runner

- delete plugin machinery (loader, install, meta, hooks) and all trigger sites
- fold first-party provider auth into static registry (provider/hooks.ts)
- remove remote instruction URL fetch, skills remote puller, TUI plugin host
- add session/context.ts: single provenance-tagged context assembly point
- add session/runner.ts: admit/context/stream/tools loop with compaction policy
- relocate audit artifacts to audit/ (FORK-AUDIT, instruction docs as evidence)

packages/opencode/test/agent/agent.test.ts

@@ -11,14 +11,13 @@ import { RuntimeFlags } from "../../src/effect/runtime-flags"
 import { Global } from "@opencode-ai/core/global"
 import { Permission } from "../../src/permission"
 import { PermissionV1 } from "@opencode-ai/core/v1/permission"
-import { Plugin } from "../../src/plugin"
 import { Provider } from "../../src/provider/provider"
 import { Skill } from "../../src/skill"
 import { Truncate } from "../../src/tool/truncate"
 
 const agentLayer = (flags: Partial<RuntimeFlags.Info> = {}) =>
   LayerNode.compile(
-    LayerNode.group([Agent.node, Plugin.node, Provider.node, Auth.node, Config.node, Skill.node, RuntimeFlags.node]),
+    LayerNode.group([Agent.node, Provider.node, Auth.node, Config.node, Skill.node, RuntimeFlags.node]),
     [[RuntimeFlags.node, RuntimeFlags.layer(flags)]],
   )