feat(coding-agent): set PI_CODING_AGENT=true env var at startup

closes #2868

packages/coding-agent/src/cli.ts

@@ -6,6 +6,7 @@
  * Test with: npx tsx src/cli-new.ts [args...]
  */
 process.title = "pi";
+process.env.PI_CODING_AGENT = "true";
 process.emitWarning = (() => {}) as typeof process.emitWarning;
 
 import { EnvHttpProxyAgent, setGlobalDispatcher } from "undici";