Fix host CWD leaking into non-local terminal backends

When using Modal, Docker, SSH, or Singularity as the terminal backend
from the CLI, the agent resolved cwd: "." to the host machine's local
path (e.g. /Users/rewbs/code/hermes-agent) and passed it to the remote
sandbox, where it doesn't exist. All commands failed with "No such file
or directory".

Root cause: cli.py unconditionally resolved "." to os.getcwd() and wrote
it to TERMINAL_CWD regardless of backend type. Every tool then used that
host-local path as the working directory inside the remote environment.

Fixes:
- cli.py: only resolve "." to os.getcwd() for the local backend. For all
  remote backends (ssh, docker, modal, singularity), leave TERMINAL_CWD
  unset so the tool layer uses per-backend defaults (/root, /, ~, etc.)
- terminal_tool.py: added sanity check -- if TERMINAL_CWD contains a
  host-local prefix (/Users/, /home/, C:\) for a non-local backend, log
  a warning and fall back to the backend's default
- terminal_tool.py: SSH default CWD is now ~ instead of os.getcwd()
- file_operations.py: last-resort CWD fallback changed from os.getcwd()
  to "/" so host paths never leak into remote file operations

tools/file_operations.py

@@ -257,9 +257,12 @@ class ShellFileOperations(FileOperations):
             cwd: Working directory (defaults to env's cwd or current directory)
         """
         self.env = terminal_env
-        # Determine cwd from various possible sources
+        # Determine cwd from various possible sources.
+        # IMPORTANT: do NOT fall back to os.getcwd() -- that's the HOST's local
+        # path which doesn't exist inside container/cloud backends (modal, docker).
+        # If nothing provides a cwd, use "/" as a safe universal default.
         self.cwd = cwd or getattr(terminal_env, 'cwd', None) or \
-                   getattr(getattr(terminal_env, 'config', None), 'cwd', None) or os.getcwd()
+                   getattr(getattr(terminal_env, 'config', None), 'cwd', None) or "/"
         
         # Cache for command availability checks
         self._command_cache: Dict[str, bool] = {}