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/core/slash-commands.ts

@@ -1,3 +1,4 @@
+import { APP_NAME } from "../config.js";
 import type { SourceInfo } from "./source-info.js";
 
 export type SlashCommandSource = "extension" | "prompt" | "skill";
@@ -35,5 +36,5 @@ export const BUILTIN_SLASH_COMMANDS: ReadonlyArray<BuiltinSlashCommand> = [
 	{ name: "compact", description: "Manually compact the session context" },
 	{ name: "resume", description: "Resume a different session" },
 	{ name: "reload", description: "Reload keybindings, extensions, skills, prompts, and themes" },
-	{ name: "quit", description: "Quit pi" },
+	{ name: "quit", description: `Quit ${APP_NAME}` },
 ];