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/opencode2-config.ts

@@ -1,5 +1,6 @@
 import { readFileSync } from "node:fs";
 import { resolve } from "node:path";
+import { normalizeMcpConfig } from "./mcp-config.js";
 
 // A conversation Location can be outside the checkout. Load only the trusted
 // service assets explicitly, rather than relying on its current directory.
@@ -11,6 +12,7 @@ export function serviceOpencodeConfig(input: Record<string, unknown>) {
   return {
     ...input,
     snapshots: false,
+    mcp: normalizeMcpConfig(input.mcp),
     plugins: ["*", ...((input.plugins ?? []) as unknown[]), resolve(".opencode/tjwater-plugin")],
     skills: [resolve(process.env.OPENCODE_SKILLS_ROOT_DIR ?? ".opencode/skills")],
     agents: { ...agents, instruction: {