feat(agent): unify MCP tools and adopt Responses delivery

Move project and host queries into native MCP with host-owned OAuth/context,
while retaining memory, skills, actions and publication plugins. Add nearby-model
queries and explicit chat/task/learning model defaults.

Adopt Responses HTTP/SSE and native steer/queue inbox delivery, preserving scoped
session ownership, pending input edits and investigation communication.

Disable local-provider discovery: Server /health allowed native vLLM to probe
/v1/models on port 8000. A native before/after regression prevents recurrence.

Validation: 170 focused Bun tests; service/plugin typechecks; native discovery,
provider-discovery and input-inbox smoke checks; no paid model inference.

.opencode/test-fixtures/task-agent-probe-plugin/index.ts

@@ -0,0 +1,15 @@
+// Loaded only by the opt-in isolated task-agent smoke test, never by the service.
+import { Plugin, Rpc } from "@opencode/plugin";
+import * as Schema from "effect/Schema";
+import { z } from "zod";
+
+export default Plugin.define({ id: "test.task-agent-probe", async setup(ctx) {
+  await ctx.rpc.register(Rpc.define({ id: "test.task-agent-probe", methods: {
+    subagent_input: { input: z.object({}), output: z.array(z.string()) },
+  }, events: {} }), { subagent_input: async () => {
+    const tool = (await ctx.tool.list()).find((item) => item.id === "subagent");
+    if (!tool) throw new Error("native subagent tool missing");
+    const input = Schema.isSchema(tool.input) ? Schema.toJsonSchemaDocument(tool.input).schema : tool.input;
+    return Object.keys((input as { properties?: Record<string, unknown> }).properties ?? {});
+  } });
+} });