fix(coding-agent): redo Bun package manager node_modules handling (#3998)

* Revert "fix(coding-agent): use alternate logic to find Bun's node_modules (#3861)"

This reverts commit c241c6d6d0.  The logic
is faulty: the original strategy of looking for node_modules by asking
the package manager is not incorrect even on bun. Instead, it should
learn a different method of asking the package manager for node_modules
when the *package manager* is bun, not the *runtime*.

* feat(coding-agent): detect bun as package manager and use alternate root query

When `"npmCommand": ["bun"]` is configured in settings.json, pi fails to
start because it invokes `bun root -g`, which doesn't exist:

    error: Failed to run bun root -g: error: Script not found "root"

Add a (simple) check for Bun being used as package manager, and instead
build the relative path starting from Bun's bin directory.

packages/coding-agent/src/core/package-manager.ts

@@ -27,7 +27,7 @@ import type { Readable } from "node:stream";
 import { globSync } from "glob";
 import ignore from "ignore";
 import { minimatch } from "minimatch";
-import { CONFIG_DIR_NAME, isBunRuntime } from "../config.js";
+import { CONFIG_DIR_NAME } from "../config.js";
 import { type GitSource, parseGitUrl } from "../utils/git.js";
 import { canonicalizePath, isLocalPath } from "../utils/paths.js";
 import { isStdoutTakenOver } from "./output-guard.js";
@@ -1843,8 +1843,9 @@ export class DefaultPackageManager implements PackageManager {
 		if (this.globalNpmRoot && this.globalNpmRootCommandKey === commandKey) {
 			return this.globalNpmRoot;
 		}
-		if (isBunRuntime) {
-			const binDir = this.runCommandSync("bun", ["pm", "bin", "-g"]).trim();
+		const isBunPackageManager = npmCommand.command === "bun";
+		if (isBunPackageManager) {
+			const binDir = this.runNpmCommandSync(["pm", "bin", "-g"]).trim();
 			this.globalNpmRoot = join(dirname(binDir), "install", "global", "node_modules");
 		} else {
 			this.globalNpmRoot = this.runNpmCommandSync(["root", "-g"]).trim();