examples: thread env and stdin through the worker-javascript exec route

The exec handler now forwards env and stdin from the request body into
runtime.exec, so a posted module can read process.env and process.stdin.
Document the extended request shape and add a smoke-test recipe that
pipes stdin and reads an env value.

Author: aron-cf

examples/worker-javascript/README.md

@@ -86,7 +86,7 @@ npm run seed:r2 --workspace @example/computer-worker-javascript
 PUT  /c/<name>/file/workspace/<path>   raw body → writeFile at /workspace/<path>
 GET  /c/<name>/file/workspace/<path>   octet-stream of /workspace/<path>
                                        (any path outside /workspace returns 400)
-POST /c/<name>/exec                    { source, input?, cwd? }
+POST /c/<name>/exec                    { source, input?, cwd?, env?, stdin? }
                                        cwd defaults to /workspace
                                        → JSON { status, exitCode, stdout, stderr, value }
 ```
@@ -120,8 +120,18 @@ curl -X POST http://127.0.0.1:8787/c/demo/exec \
 curl -X POST http://127.0.0.1:8787/c/demo/exec \
   -H 'content-type: application/json' \
   -d '{"source":"export default (input) => input.n * 2;","input":{"n":21}}'
+
+curl -X POST http://127.0.0.1:8787/c/demo/exec \
+  -H 'content-type: application/json' \
+  -d '{"source":"export default async () => { let s = \"\"; for await (const c of process.stdin) s += new TextDecoder().decode(c); return process.env.WHO + \":\" + s; };","env":{"WHO":"demo"},"stdin":"piped"}'
 ```
 
+`env` populates `process.env` (only the values you pass; the host
+environment is never exposed), and `stdin` is readable through
+`process.stdin`. `console.log` / `console.error` and
+`process.stdout` / `process.stderr` writes come back as the result's
+`stdout` and `stderr`.
+
 ## Layout
 
 ```

examples/worker-javascript/src/index.ts

@@ -25,6 +25,8 @@ interface ExecRequest {
   source?: string;
   input?: WorkspaceRuntimeValue;
   cwd?: string;
+  env?: Record<string, string>;
+  stdin?: string;
 }
 
 const MOUNT_ROOT = "/workspace";
@@ -130,6 +132,8 @@ async function handleExec(request: Request, env: Env, name: string): Promise<Res
       backend: "worker-javascript",
       cwd: body.cwd,
       input: body.input,
+      env: body.env,
+      stdin: body.stdin,
       encoding: "utf8",
     });
     const result = await handle.result();