Fix: Detect Bun global install via path check (#8004)

## Summary
Restores ability to detect when Codex is installed globally via **Bun**,
which was broken by c3e4f920b4. Fixes
#8003.

Instead of relying on `npm_config_user_agent` (which is only set when
running via `bunx` or `bun run`), this adds a path-based check to see if
the CLI wrapper is located in Bun's global installation directory.

## Regression Context
Commit `c3e4f920b4e965085164d6ee0249a873ef96da77` removed the
`BUN_INSTALL` environment variable checks to prevent false positives.
However, this caused false negatives for genuine Bun global installs
because `detectPackageManager()` defaults to NPM when no signal is
found.

## Changes
- Updated `codex-cli/bin/codex.js` to check if `__dirname` contains
`.bun/install/global` (handles both POSIX and Windows paths).

## Verification
Verified by performing a global install of the patched CLI (v0.69.0 to
trigger the update prompt):

1. Packed the CLI using `npm pack` in `codex-cli/` to create a release
tarball.
2. Installed globally via Bun: `bun install -g
$(pwd)/openai-codex-0.0.0-dev.tgz`.
3. Ran `codex`, confirmed it detected Bun (banner showed `bun install -g
@openai/codex`), selected "Update now", and verified it correctly
spawned `bun install -g` instead of `npm`.
4. Confirmed the upgrade completed successfully using Bun.
<img width="1038" height="813" alt="verifying installation via bun"
src="https://github.com/user-attachments/assets/00c9301a-18f1-4440-aa95-82ccffba896c"
/>
5. Verified installations via npm are unaffected.
<img width="2090" height="842" alt="verifying installation via npm"
src="https://github.com/user-attachments/assets/ccb3e031-b85c-4bbe-bac7-23b087c5b844"
/>

codex-cli/bin/codex.js

@@ -95,6 +95,14 @@ function detectPackageManager() {
     return "bun";
   }
 
+
+  if (
+    __dirname.includes(".bun/install/global") ||
+    __dirname.includes(".bun\\install\\global")
+  ) {
+    return "bun";
+  }
+
   return userAgent ? "npm" : null;
 }