refactor: extract getCwd() helper to deduplicate working directory resolution (#403)

## Summary

- Extracts the duplicated working-directory resolution expression
(`core.getInput('working_directory') || core.getInput('install_dir') ||
process.cwd()`) into a `getCwd()` helper function.
- Replaces the 2 identical inline expressions in `exportMiseEnv()` and
`mise()` with calls to `getCwd()`.

## Motivation

Separated from #402 to allow independent review. This is a pure refactor
with no behavioral change, intended to simplify the diff in #402 by
providing a smaller, self-contained improvement.

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

src/index.ts

@@ -82,10 +82,7 @@ async function run(): Promise<void> {
 async function exportMiseEnv(): Promise<void> {
   core.startGroup('Exporting mise environment variables')
 
-  const cwd =
-    core.getInput('working_directory') ||
-    core.getInput('install_dir') ||
-    process.cwd()
+  const cwd = getCwd()
 
   // Check if mise supports --redacted flags based on version input
   const supportsRedacted = checkMiseSupportsRedacted()
@@ -356,10 +353,7 @@ const miseLs = async (): Promise<number> => mise([`ls`])
 const miseReshim = async (): Promise<number> => mise([`reshim`, `-f`])
 const mise = async (args: string[]): Promise<number> =>
   await core.group(`Running mise ${args.join(' ')}`, async () => {
-    const cwd =
-      core.getInput('working_directory') ||
-      core.getInput('install_dir') ||
-      process.cwd()
+    const cwd = getCwd()
     const baseEnv = Object.fromEntries(
       Object.entries(process.env).filter(
         (entry): entry is [string, string] => entry[1] !== undefined
@@ -387,6 +381,14 @@ const writeFile = async (p: fs.PathLike, body: string): Promise<void> =>
 
 run()
 
+function getCwd(): string {
+  return (
+    core.getInput('working_directory') ||
+    core.getInput('install_dir') ||
+    process.cwd()
+  )
+}
+
 function miseDir(): string {
   const dir = core.getState('MISE_DIR')
   if (dir) return dir