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/tool/code-mode-integration.test.ts

@@ -3,7 +3,6 @@ import { CodeModeTool, describeCatalog } from "@/tool/code-mode"
 import { McpCatalog } from "@/mcp/catalog"
 import { Agent } from "@/agent/agent"
 import { MCP } from "@/mcp"
-import { Plugin } from "@/plugin"
 import { Session } from "@/session/session"
 import { Tool } from "@/tool/tool"
 import * as Truncate from "@/tool/truncate"
@@ -139,10 +138,6 @@ async function buildTool() {
   }
 
   const layer = Layer.mergeAll(
-    Layer.mock(Plugin.Service, {
-      trigger: ((_name: unknown, _input: unknown, output: unknown) =>
-        Effect.succeed(output)) as Plugin.Interface["trigger"],
-    }),
     Layer.mock(Truncate.Service, {
       output: (text: string) => Effect.succeed({ content: text, truncated: false as const }),
     }),