Fix Modal backend not working from CLI

Two config systems used different key names for the terminal backend:
- hermes_cli/config.py, README, and all docs use "terminal.backend"
- cli.py's env var mapping only recognized "terminal.env_type"

Users following the docs who set `backend: modal` in ~/.hermes/config.yaml
had it silently ignored -- TERMINAL_ENV always defaulted to "local".

Additionally, when no config file existed, cli.py's hardcoded defaults
overwrote any TERMINAL_ENV=modal set in .env, despite the comment saying
"env vars take precedence."

Fixes:
- cli.py now normalizes "backend" -> "env_type" (backend takes precedence)
- Defaults no longer overwrite .env when no config file terminal section exists
- hermes status reads from config as fallback when env var isn't set

Also fixes four related bugs found in the Modal/sandbox lifecycle:
- file_tools cache not cleared on sandbox cleanup (stale ops on dead sandbox)
- Global lock held during slow Modal teardown (blocked all tool calls 10-15s)
- Race condition in file_tools between existence check and access (KeyError)
- Per-task creation locks never cleaned up (memory leak)

hermes_cli/status.py

@@ -91,7 +91,16 @@ def show_status(args):
     print()
     print(color("◆ Terminal Backend", Colors.CYAN, Colors.BOLD))
     
-    terminal_env = os.getenv("TERMINAL_ENV", "local")
+    terminal_env = os.getenv("TERMINAL_ENV", "")
+    if not terminal_env:
+        # Fall back to config file value when env var isn't set
+        # (hermes status doesn't go through cli.py's config loading)
+        try:
+            from hermes_cli.config import load_config
+            _cfg = load_config()
+            terminal_env = _cfg.get("terminal", {}).get("backend", "local")
+        except Exception:
+            terminal_env = "local"
     print(f"  Backend:      {terminal_env}")
     
     if terminal_env == "ssh":