feat(v2): OpenCode agent provider

- Add OpenCodeProvider (SSE, session resume, MCP via mcp-to-opencode)
- Register opencode in factory; AGENT_PROVIDER passthrough from DB
- Host: XDG mount, NO_PROXY merge, OPENCODE_* env for opencode sessions
- Dockerfile: opencode-ai CLI; docs checklist + architecture diagram
- Skill add-opencode for v2; AgentProviderName in src/types.ts

Made-with: Cursor

src/providers/index.ts

@@ -4,3 +4,5 @@
 // needs (claude, mock) don't appear here.
 //
 // Skills add a new provider by appending one import line below.
+
+import './opencode.js';

src/providers/opencode.ts

@@ -0,0 +1,49 @@
+/**
+ * Host-side container config for the `opencode` provider.
+ *
+ * OpenCode's `opencode serve` process stores state under XDG_DATA_HOME, which
+ * we pin to a per-session host directory mounted at /opencode-xdg. The
+ * OPENCODE_* env vars tell the CLI which provider/model to use at runtime
+ * (read on the host, injected into the container). NO_PROXY / no_proxy are
+ * merged with host values so the in-container OpenCode client can talk to
+ * 127.0.0.1 even when HTTPS_PROXY is set by OneCLI.
+ */
+import fs from 'fs';
+import path from 'path';
+
+import { registerProviderContainerConfig } from './provider-container-registry.js';
+
+function mergeNoProxy(current: string | undefined, additions: string): string {
+  if (!current?.trim()) return additions;
+  const parts = new Set(
+    current
+      .split(/[\s,]+/)
+      .map((s) => s.trim())
+      .filter(Boolean),
+  );
+  for (const addition of additions.split(',')) {
+    const trimmed = addition.trim();
+    if (trimmed) parts.add(trimmed);
+  }
+  return [...parts].join(',');
+}
+
+registerProviderContainerConfig('opencode', (ctx) => {
+  const opencodeDir = path.join(ctx.sessionDir, 'opencode-xdg');
+  fs.mkdirSync(opencodeDir, { recursive: true });
+
+  const env: Record<string, string> = {
+    XDG_DATA_HOME: '/opencode-xdg',
+    NO_PROXY: mergeNoProxy(ctx.hostEnv.NO_PROXY, '127.0.0.1,localhost'),
+    no_proxy: mergeNoProxy(ctx.hostEnv.no_proxy, '127.0.0.1,localhost'),
+  };
+  for (const key of ['OPENCODE_PROVIDER', 'OPENCODE_MODEL', 'OPENCODE_SMALL_MODEL'] as const) {
+    const value = ctx.hostEnv[key];
+    if (value) env[key] = value;
+  }
+
+  return {
+    mounts: [{ hostPath: opencodeDir, containerPath: '/opencode-xdg', readonly: false }],
+    env,
+  };
+});