fix(security): sanitize workdir parameter in terminal tool backends

Shell injection via unquoted workdir interpolation in docker, singularity,
and SSH backends.  When workdir contained shell metacharacters (e.g.
~/;id), arbitrary commands could execute.

Changes:
- Add shlex.quote() at each interpolation point in docker.py,
  singularity.py, and ssh.py with tilde-aware quoting (keep ~
  unquoted for shell expansion, quote only the subpath)
- Add _validate_workdir() allowlist in terminal_tool.py as
  defense-in-depth before workdir reaches any backend

Original work by Mariano A. Nicolini (PR #5620).  Salvaged with fixes
for tilde expansion (shlex.quote breaks cd ~/path) and replaced
incomplete deny-list with strict character allowlist.

Co-authored-by: Mariano A. Nicolini <entropidelic@users.noreply.github.com>

tools/environments/docker.py

@@ -8,6 +8,7 @@ persistence via bind mounts.
 import logging
 import os
 import re
+import shlex
 import shutil
 import subprocess
 import sys
@@ -484,9 +485,13 @@ class DockerEnvironment(BaseEnvironment):
         else:
             effective_stdin = stdin_data
 
-        # docker exec -w doesn't expand ~, so prepend a cd into the command
-        if work_dir == "~" or work_dir.startswith("~/"):
-            exec_command = f"cd {work_dir} && {exec_command}"
+        # docker exec -w doesn't expand ~, so prepend a cd into the command.
+        # Keep ~ unquoted (for shell expansion) and quote only the subpath.
+        if work_dir == "~":
+            exec_command = f"cd ~ && {exec_command}"
+            work_dir = "/"
+        elif work_dir.startswith("~/"):
+            exec_command = f"cd ~/{shlex.quote(work_dir[2:])} && {exec_command}"
             work_dir = "/"
 
         assert self._container_id, "Container not started"