Remove commondir dependency, inline as commonParentDirectory()

The commondir package is a tiny, unmaintained (~2014) utility. Inline
its logic as an exported commonParentDirectory() function in path.ts,
removing both the runtime dep and @types/commondir. Added parity tests
covering the original package's test cases.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ryancbahan

packages/cli-kit/package.json

@@ -121,7 +121,6 @@
     "chalk": "5.4.1",
     "change-case": "4.1.2",
     "color-json": "3.0.5",
-    "commondir": "1.0.1",
     "conf": "11.0.2",
     "deepmerge": "4.3.1",
     "dotenv": "16.4.7",
@@ -165,7 +164,6 @@
     "zod": "3.24.4"
   },
   "devDependencies": {
-    "@types/commondir": "^1.0.0",
     "@types/diff": "^5.2.3",
     "@types/fs-extra": "9.0.13",
     "@types/gradient-string": "^1.1.2",

packages/cli-kit/src/public/node/path.test.ts

@@ -1,4 +1,4 @@
-import {relativizePath, normalizePath, cwd, sniffForPath} from './path.js'
+import {relativizePath, normalizePath, cwd, sniffForPath, commonParentDirectory} from './path.js'
 import {describe, test, expect} from 'vitest'
 
 describe('relativize', () => {
@@ -25,6 +25,40 @@ describe('cwd', () => {
   })
 })
 
+describe('commonParentDirectory', () => {
+  // Parity tests with the original 'commondir' npm package (v1.0.1)
+  test('finds common parent for paths sharing a prefix', () => {
+    expect(commonParentDirectory('/foo', '/foo/bar')).toBe('/foo')
+    expect(commonParentDirectory('/foo/bar', '/foo//bar/baz')).toBe('/foo/bar')
+  })
+
+  test('finds deepest common ancestor', () => {
+    expect(commonParentDirectory('/a/b/c', '/a/b')).toBe('/a/b')
+    expect(commonParentDirectory('/a/b', '/a/b/c/d/e')).toBe('/a/b')
+  })
+
+  test('returns root when paths diverge at top level', () => {
+    expect(commonParentDirectory('/x/y/z/w', '/xy/z')).toBe('/')
+  })
+
+  test('handles Windows-style paths', () => {
+    expect(commonParentDirectory('X:\\foo', 'X:\\\\foo\\bar')).toBe('X:/foo')
+    expect(commonParentDirectory('X:\\a\\b\\c', 'X:\\a\\b')).toBe('X:/a/b')
+  })
+
+  test('returns root for completely divergent Windows paths', () => {
+    expect(commonParentDirectory('X:\\x\\y\\z\\w', '\\\\xy\\z')).toBe('/')
+  })
+
+  test('returns root for single-component paths', () => {
+    expect(commonParentDirectory('/', '/')).toBe('/')
+  })
+
+  test('handles identical paths', () => {
+    expect(commonParentDirectory('/a/b/c', '/a/b/c')).toBe('/a/b/c')
+  })
+})
+
 describe('sniffForPath', () => {
   test('returns the path if provided', () => {
     // Given

packages/cli-kit/src/public/node/path.ts

@@ -1,4 +1,3 @@
-import commondir from 'commondir'
 import {
   relative,
   dirname as patheDirname,
@@ -106,6 +105,23 @@ export function parsePath(path: string): {root: string; dir: string; base: strin
   return parse(path)
 }
 
+/**
+ * Returns the longest common parent directory of two absolute paths.
+ *
+ * @param first - First absolute path.
+ * @param second - Second absolute path.
+ * @returns The common parent directory, or '/' if they share only the root.
+ */
+export function commonParentDirectory(first: string, second: string): string {
+  const firstParts = first.split(/\/+|\\+/)
+  const secondParts = second.split(/\/+|\\+/)
+  let i = 0
+  while (i < firstParts.length && i < secondParts.length && firstParts[i] === secondParts[i]) {
+    i++
+  }
+  return i > 1 ? firstParts.slice(0, i).join('/') : '/'
+}
+
 /**
  * Given an absolute filesystem path, it makes it relative to
  * the current working directory. This is useful when logging paths
@@ -117,7 +133,7 @@ export function parsePath(path: string): {root: string; dir: string; base: strin
  * @returns Relativized path.
  */
 export function relativizePath(path: string, dir: string = cwd()): string {
-  const result = commondir([path, dir])
+  const result = commonParentDirectory(path, dir)
   const relativePath = relative(dir, path)
   // eslint-disable-next-line @typescript-eslint/ban-ts-comment
   // @ts-ignore