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/extensions/loader.ts

@@ -21,7 +21,7 @@ import * as _bundledPiTui from "@mariozechner/pi-tui";
 import * as _bundledTypebox from "typebox";
 import * as _bundledTypeboxCompile from "typebox/compile";
 import * as _bundledTypeboxValue from "typebox/value";
-import { getAgentDir, isBunBinary } from "../../config.js";
+import { CONFIG_DIR_NAME, getAgentDir, isBunBinary } from "../../config.js";
 // NOTE: This import works because loader.ts exports are NOT re-exported from index.ts,
 // avoiding a circular dependency. Extensions can import from @mariozechner/pi-coding-agent.
 import * as _bundledPiCodingAgent from "../../index.js";
@@ -576,8 +576,8 @@ export async function discoverAndLoadExtensions(
 		}
 	};
 
-	// 1. Project-local extensions: cwd/.pi/extensions/
-	const localExtDir = path.join(cwd, ".pi", "extensions");
+	// 1. Project-local extensions: cwd/${CONFIG_DIR_NAME}/extensions/
+	const localExtDir = path.join(cwd, CONFIG_DIR_NAME, "extensions");
 	addPaths(discoverExtensionsInDir(localExtDir));
 
 	// 2. Global extensions: agentDir/extensions/