feat(connectd): companion cwd (writable session dir for the WhatsApp bridge)

companion.cwd (created if missing; ~/$HOME expanded) sets the sidecar's working directory so the
WhatsApp bridge writes its session store to a writable path (~/.neuron/whatsapp), never the read-only
app bundle. Typecheck clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/bridge.ts

@@ -5,6 +5,8 @@ import { UnauthorizedError } from "@modelcontextprotocol/sdk/client/auth.js";
 import { createHash } from "node:crypto";
 import { join, dirname } from "node:path";
 import { spawn, type ChildProcess } from "node:child_process";
+import { mkdirSync } from "node:fs";
+import { homedir } from "node:os";
 import type { ConnectorsConfig, ServerConfig } from "./config.js";
 import { saveConfig } from "./config.js";
 import { KeychainOAuthProvider, takePendingAuthUrl } from "./oauth.js";
@@ -190,10 +192,15 @@ export class ConnectorBridge {
     };
     if (await ping()) return; // already running (this session or a prior one)
     if (this.companions.has(id)) return; // we already started it
+    let cwd: string | undefined;
+    if (comp.cwd) {
+      cwd = comp.cwd.replace(/^~(?=\/|$)/, homedir()).replace(/^\$HOME(?=\/|$)/, homedir());
+      try { mkdirSync(cwd, { recursive: true }); } catch { /* best effort — spawn will surface a real failure */ }
+    }
     const child = spawn(
       resolveRuntimeCommand(comp.command),
       (comp.args ?? []).map(resolveBundledArg),
-      { env: { ...process.env, ...(comp.env ?? {}) } as Record<string, string>, stdio: "ignore" },
+      { env: { ...process.env, ...(comp.env ?? {}) } as Record<string, string>, stdio: "ignore", cwd },
     );
     this.companions.set(id, child);
     child.on("exit", () => this.companions.delete(id));

src/config.ts

@@ -20,6 +20,10 @@ export interface ServerConfig {
     args?: string[];
     env?: Record<string, string>;
     healthUrl?: string;
+    // Working directory for the sidecar (created if missing). The WhatsApp bridge writes its session
+    // store relative to cwd, so this must be writable (e.g. ~/.neuron/whatsapp) — never the read-only
+    // app bundle. "~" / "$HOME" at the start are expanded to the user's home.
+    cwd?: string;
   };
   // http (Phase 3)
   url?: string;