[Windows] False "Vercel CLI not installed" warning when username contains non-ASCII characters

Bug: The session startup hook reports "IMPORTANT: The Vercel CLI is not installed"
even though Vercel CLI is correctly installed.

Root cause: resolveBinaryFromPath() in session-start-profiler.mjs uses
accessSync(path, X_OK) to locate the Vercel binary. On Windows, when the user's
home directory path contains non-ASCII characters (e.g. accented letters like "é"
in "Andrés"), the PATH entries are decoded with a different encoding (Windows
code page, e.g. CP850), causing accessSync to receive a corrupted path that
doesn't exist — resulting in ENOENT and a false negative.

Environment:

• OS: Windows 10 (10.0.19045)
• Username: contains accented character (é)
• npm global path: C:\Users\Andrés\AppData\Roaming\npm\vercel.cmd
• Vercel CLI version: 50.39.0 (verified via vercel --version)
• Plugin version: 0.24.0

Reproduction: Install Vercel CLI globally on Windows with a username
containing non-ASCII characters. Every Claude Code session startup shows
the "not installed" warning.

Workaround: None currently. The CLI works fine — only the detection fails.

Suggested fix: Fall back to spawning vercel --version directly (via
execFileSync('vercel', ...)) when resolveBinaryFromPath returns null on
Windows, as the shell resolves .cmd extensions correctly regardless of path encoding.

Start by reading resolveBinaryFromPath() in session-start-profiler.mjs and trace how Windows PATH entries reach accessSync(). Reproduce the session startup on Windows with a non-ASCII username, then verify that an installed Vercel CLI no longer triggers the warning while normal missing-CLI detection still works.