fix(coding-agent,ai): fallback to /proc/self/environ in Bun sandbox (#3801)

Bun compiled binaries have an empty process.env when running inside
sandbox environments (e.g. nono on Linux/macOS). This broke API key
detection and model discovery because all process.env.* lookups returned
undefined.

- Add restoreSandboxEnv() helper that reads /proc/self/environ when Bun
  is detected and process.env is empty, populating process.env before
  any other code runs (coding-agent/src/bun/cli.ts entry point)
- Add getProcEnv() fallback in env-api-keys.ts for direct @mariozechner/pi-ai
  consumers that may not go through the coding-agent entry point
- Add unit tests for restoreSandboxEnv

packages/coding-agent/src/bun/cli.ts

@@ -4,5 +4,9 @@ import { APP_NAME } from "../config.js";
 process.title = APP_NAME;
 process.emitWarning = (() => {}) as typeof process.emitWarning;
 
+import { restoreSandboxEnv } from "./restore-sandbox-env.js";
+
+restoreSandboxEnv();
+
 await import("./register-bedrock.js");
 await import("../cli.js");