Fix resolveGrokBin() ENOENT on Windows

[irdatrais]

Fixes #2

Problem

On Windows (Git Bash / MSYS2), grok-companion.mjs fails every task with:

Failed to launch grok: spawn /c/Users/<user>/.grok/bin/grok ENOENT

resolveGrokBin() resolves the binary path via which grok (or the ~/.grok/bin/grok fallback) and validates it with existsSync(). The actual installed binary is grok.exe, but:

• which grok (Git Bash) returns the path without the .exe suffix
• existsSync() on Windows resolves that bare path leniently and returns true even though the file on disk is grok.exe
• spawn() does not share that leniency and requires the exact filename, so it throws ENOENT

Setting GROK_BIN manually didn't help either, since the same lenient existsSync() check was used for that path too. The setup subcommand also silently masked this: it reported ok: true with a resolved binary path, while grokVersion() (which spawns that same unresolved path) quietly returned null.

Fix

Added resolveSpawnable(), which on win32 checks for a .exe/.cmd/.bat suffix when the bare path doesn't spawn cleanly, and applied it to all three resolution sources (GROK_BIN, which, and the default fallback path).

Verification

Tested on Windows with GROK_BIN unset:

Before:

{ "ok": true, "binary": "...\grok", "version": null }

task run → Failed to launch grok: spawn ...\grok ENOENT

After:

{ "ok": true, "binary": "...\grok.exe", "version": "grok 0.2.118 (1e1687c1cf) [stable]" }

task --read "..." → runs end-to-end successfully.

No changes to non-Windows behavior — resolveSpawnable() is a no-op passthrough (just existsSync) on other platforms.