fix(coding-agent): route remaining hardcoded pi branding through APP_NAME (#3583)

Swaps hardcoded "pi" / ".pi" / "π" for the existing APP_NAME and
CONFIG_DIR_NAME extension points at four sites the original APP_NAME
rollout missed:

- /quit slash-command description
- process.title (cli.ts and bun/cli.ts)
- project-local extensions dir in loader.ts

For the terminal title, adds an APP_TITLE export in config.ts that
keeps pi's "π" glyph when piConfig.name is unset and falls back to
APP_NAME otherwise. Presence of piConfig.name acts as the "has been
rebranded" signal, so pi's own package.json drops the redundant
"name": "pi" (the existing `|| "pi"` fallback in config.ts makes this
a no-op for APP_NAME).

No behavior change for pi itself: process.title, /quit description,
terminal title prefix, and project-local extensions dir all resolve
to the same values as before.

closes #3476

packages/coding-agent/src/cli.ts

@@ -5,13 +5,14 @@
  *
  * Test with: npx tsx src/cli-new.ts [args...]
  */
-process.title = "pi";
+import { EnvHttpProxyAgent, setGlobalDispatcher } from "undici";
+import { APP_NAME } from "./config.js";
+import { main } from "./main.js";
+
+process.title = APP_NAME;
 process.env.PI_CODING_AGENT = "true";
 process.emitWarning = (() => {}) as typeof process.emitWarning;
 
-import { EnvHttpProxyAgent, setGlobalDispatcher } from "undici";
-import { main } from "./main.js";
-
 setGlobalDispatcher(new EnvHttpProxyAgent());
 
 main(process.argv.slice(2));