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

cli.py

@@ -173,10 +173,19 @@ def load_cli_config() -> Dict[str, Any]:
     if "backend" in terminal_config:
         terminal_config["env_type"] = terminal_config["backend"]
     
-    # Handle special cwd values: "." or "auto" means use current working directory
+    # Handle special cwd values: "." or "auto" means use current working directory.
+    # Only resolve to the host's CWD for the local backend where the host
+    # filesystem is directly accessible.  For ALL remote/container backends
+    # (ssh, docker, modal, singularity), the host path doesn't exist on the
+    # target -- remove the key so terminal_tool.py uses its per-backend default.
     if terminal_config.get("cwd") in (".", "auto", "cwd"):
-        terminal_config["cwd"] = os.getcwd()
-        defaults["terminal"]["cwd"] = terminal_config["cwd"]
+        effective_backend = terminal_config.get("env_type", "local")
+        if effective_backend == "local":
+            terminal_config["cwd"] = os.getcwd()
+            defaults["terminal"]["cwd"] = terminal_config["cwd"]
+        else:
+            # Remove so TERMINAL_CWD stays unset → tool picks backend default
+            terminal_config.pop("cwd", None)
     
     env_mappings = {
         "env_type": "TERMINAL_ENV",