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/config.ts

@@ -184,7 +184,9 @@ export function getBundledInteractiveAssetPath(name: string): string {
 
 const pkg = JSON.parse(readFileSync(getPackageJsonPath(), "utf-8"));
 
-export const APP_NAME: string = pkg.piConfig?.name || "pi";
+const piConfigName: string | undefined = pkg.piConfig?.name;
+export const APP_NAME: string = piConfigName || "pi";
+export const APP_TITLE: string = piConfigName ? APP_NAME : "π";
 export const CONFIG_DIR_NAME: string = pkg.piConfig?.configDir || ".pi";
 export const VERSION: string = pkg.version;