fix(update): merge custom env with process.env in spawn

When the update runner passes custom env vars (like CLAWDBOT_UPDATE_IN_PROGRESS),
the current code uses `env ?? process.env` which replaces the entire environment
instead of merging — losing PATH, HOME, etc.

This causes the doctor step to fail with 'node: No such file or directory'.

Fix: merge custom env with process.env instead of replacing it.

src/process/exec.ts

@@ -66,7 +66,7 @@ export async function runCommandWithTimeout(
     const child = spawn(argv[0], argv.slice(1), {
       stdio: [hasInput ? "pipe" : "inherit", "pipe", "pipe"],
       cwd,
-      env: env ?? process.env,
+      env: env ? { ...process.env, ...env } : process.env,
     });
     let stdout = "";
     let stderr = "";