fix(gateway,terminal): expand shell tilde in terminal.cwd before subprocess

Commit 3c42064e made config.yaml the single source of truth for
TERMINAL_CWD, but the config bridge passes cwd values verbatim to
os.environ. When a user sets terminal.cwd: ~/ in config.yaml, the
literal string '~/'' reaches subprocess.Popen, which the kernel
rejects because it does not expand shell tilde syntax.

This patch adds three defensive layers:

1. gateway/run.py — expanduser at config bridge time so TERMINAL_CWD
   is always an absolute path.

2. tools/terminal_tool.py — expanduser when reading TERMINAL_CWD in
   _get_env_config(), guarding against stale or manually-set env vars.

3. tools/environments/local.py — expanduser in LocalEnvironment before
   passing cwd to subprocess.Popen, the final safety net.

Includes regression tests in test_config_cwd_bridge.py for nested
terminal.cwd, top-level cwd alias, and precedence ordering.

Refs: 3c42064e

tools/environments/local.py

@@ -305,6 +305,8 @@ class LocalEnvironment(BaseEnvironment):
     """
 
     def __init__(self, cwd: str = "", timeout: int = 60, env: dict = None):
+        if cwd:
+            cwd = os.path.expanduser(cwd)
         super().__init__(cwd=cwd or os.getcwd(), timeout=timeout, env=env)
         self.init_session()