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/shell.test.ts

@@ -16,7 +16,6 @@ import { Truncate } from "@/tool/truncate"
 import { SessionID, MessageID } from "../../src/session/schema"
 import { CrossSpawnSpawner } from "@opencode-ai/core/cross-spawn-spawner"
 import { FSUtil } from "@opencode-ai/core/fs-util"
-import { Plugin } from "../../src/plugin"
 import { testEffect } from "../lib/effect"
 import { Tool } from "@/tool/tool"
 import { RuntimeFlags } from "@/effect/runtime-flags"
@@ -27,7 +26,6 @@ const shellLayer = Layer.mergeAll(
     LayerNode.group([
       CrossSpawnSpawner.node,
       FSUtil.node,
-      Plugin.node,
       Truncate.node,
       Config.node,
       Agent.node,