fix: Windows CLI detection and CLAUDE_CLI_PATH env var

[bluzername]

Problem

On Windows the MCP server cannot find Claude CLI because:

1. It only checks ~/.claude/local/claude which is Unix-specific path, Windows npm install puts claude.cmd in AppData
2. The CLAUDE_CLI_PATH env var mentioned in CLAUDE.md is not actually implemented (only CLAUDE_CLI_NAME exists)
3. Windows .cmd and .exe extensions are not considered when looking for the binary

This makes the server completely unusable on Windows npm installations.

What I changed

Updated findClaudeCli() in src/server.ts to handle Windows properly:

1. Added CLAUDE_CLI_PATH env var - checks this first, validates file exists, warns if not found but continues to other methods. This is what users expect based on the CLAUDE.md documentation
2. Windows extension handling - on win32 platform, also checks for .cmd and .exe variants when looking at ~/.claude/local/claude
3. Windows npm global paths - checks AppData/Roaming/npm/claude.cmd and .exe which is where npm global installs go on Windows
4. Kept backward compatibility - CLAUDE_CLI_NAME still works same as before, CLAUDE_CLI_PATH just has higher priority

Also added tests for the new CLAUDE_CLI_PATH behavior (existing unit test file already had broken mocks before this change, so only added tests that follow same pattern).

How to use on Windows

Either:

{
  "env": {
    "CLAUDE_CLI_PATH": "C:\\Users\\me\\AppData\\Roaming\\npm\\claude.cmd"
  }
}

Or just let the auto-detection find it in the npm global path.

Testing

• npm run build passes clean
• e2e tests pass
• validation tests pass
• Added 2 new unit tests for CLAUDE_CLI_PATH (set+exists, set+missing)

Closes #39