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/provider/transform.test.ts

@@ -571,11 +571,6 @@ describe("ProviderTransform.options - gpt-5 textVerbosity", () => {
         },
         provider: { id: "azure", options: { useCompletionUrls: true } } as any,
         auth: undefined,
-        plugin: {
-          trigger: (_name: string, _input: unknown, output: unknown) => Effect.succeed(output),
-          list: () => Effect.succeed([]),
-          init: () => Effect.void,
-        } as any,
         flags: { outputTokenMax: 32_000, client: "test" } as any,
         isWorkflow: false,
       }),