fix(tools): portable base64 encoding for image reading on macOS

[CharlieKerfoot]

Problem

_read_image() in tools/file_operations.py called base64 -w 0 to
produce unwrapped base64 output. The -w 0 flag is GNU coreutils syntax
and is not supported by macOS base64. On macOS with the local terminal
backend this caused every read_file call on an image to fail silently
(exit code != 0), returning a generic error instead of the base64 content.
Docker/Modal/SSH backends running Linux were unaffected.

Fix

Replaced base64 -w 0 <path> with base64 <path> | tr -d '\n'.

Both GNU and macOS base64 wrap output by default; piping through
tr -d '\n' strips the newlines without needing any platform detection.
This is portable across all terminal backends (local macOS, local Linux,
Docker, Modal, SSH to macOS or Linux hosts).

Testing

On macOS with the local terminal backend:

1. hermes chat -q "Read this image: /path/to/some/image.png"
2. Verify the agent receives base64 content instead of a read error