feat(agent): add external MCP client support

Reuse OpenCode 2 HTTP/stdio clients with per-session read-only grants,
native approval and sanitized owner-scoped MCP status. Narrow the
environment inherited by trusted local MCP processes.

Validated in an isolated checkout with bun run check and 20 focused tests.

src/runtime/opencode.ts

@@ -7,6 +7,7 @@ import { resolve } from "node:path";
 import { realpath, rm } from "node:fs/promises";
 import { createConversationWorkspace, resolveConversationWorkspace } from "./conversationWorkspace.js";
 import { serviceOpencodeConfig } from "./opencode2-config.js";
+import { mcpReadonlyTools, mcpRuntimeReportSchema } from "./mcp-config.js";
 
 import { config } from "../config.js";
 import { logger } from "../logger.js";
@@ -95,6 +96,25 @@ export class OpencodeRuntimeAdapter {
     return { healthy: true, version: info.version };
   }
 
+  mcpReadonlyTools(): string[] {
+    return mcpReadonlyTools(buildOpencodeConfig());
+  }
+
+  async mcpStatus(sessionId: string) {
+    const client = await this.ensureClient();
+    const session = await client.session.get({ sessionID: sessionId });
+    const directory = resolve(session.location.directory) === process.cwd() ? process.cwd()
+      : await resolveConversationWorkspace(session.location.directory, config.CONVERSATION_WORKSPACE_ROOT);
+    return this.readMcpReport(client, directory);
+  }
+
+  private async readMcpReport(client: OpencodeClient, directory: string) {
+    const response = await client.rpc.call({
+      rpcID: "tjwater.tools", method: "mcp", input: {}, location: { directory },
+    }, { signal: AbortSignal.timeout(15_000) });
+    return mcpRuntimeReportSchema.parse(response.output);
+  }
+
   async resolveAvailableModel(preferredModelIds: string[]): Promise<string> {
     const client = await this.ensureClient();
     // V2 2.0.24 model.list alone does not await a cold Location's plugin setup.
@@ -126,6 +146,8 @@ export class OpencodeRuntimeAdapter {
       if (!tools?.some((tool) => tool.id === "shell" && tool.sandboxed)) throw new Error("OpenCode shell sandbox replacement is missing");
       const actualRoot = tools.find((tool) => tool.id === "shell")?.workspaceRoot;
       if (!actualRoot || await realpath(actualRoot) !== await realpath(config.CONVERSATION_WORKSPACE_ROOT)) throw new Error("OpenCode and Agent conversation workspace roots do not match");
+      const mcp = await this.readMcpReport(client, session.location.directory);
+      if (mcp.configuration_errors.length) throw new Error("MCP configuration invalid");
       const models = await client.model.list({ location: { directory: session.location.directory } });
       const model = models.data.find((model) => `${model.providerID}/${model.id}` === config.OPENCODE_MODEL);
       if (!model?.enabled || !model.capabilities.tools) {