feat: Add get_proxy_host_logs tool for reading nginx proxy host logs

Reads nginx access/error logs directly from a mounted NPM log directory,
enabling agents to debug proxy issues without SSH access. Requires mounting
NPM's /data/logs volume and setting NPM_LOG_DIR.

Also includes a feature request PRD for proposing a native log API upstream.

src/npm_mcp/exceptions.py

@@ -31,3 +31,9 @@ class NpmApiError(NpmClientError):
     def __init__(self, message: str, status_code: int | None = None):
         super().__init__(message)
         self.status_code = status_code
+
+
+class NpmLogError(NpmClientError):
+    """Raised when log file operations fail."""
+
+    pass