fix(terminal): bridge docker_env config to TERMINAL_DOCKER_ENV

Problem: terminal.docker_env set in config.yaml was silently ignored.
Docker containers never received the user-specified env vars.

Root cause: docker_env was missing from all three config→env bridging
maps (cli.py env_mappings, gateway/run.py _terminal_env_map,
hermes_cli/config.py _config_to_env_sync) and from the terminal_tool
_get_env_config() reader. _create_environment() consumed the key from
container_config correctly, but it was always {} because TERMINAL_DOCKER_ENV
was never set.

Also extend the list-serialisation branches in cli.py and gateway/run.py
to handle dict values via json.dumps (lists already used json.dumps;
plain str() on a dict produces undecodable output).

Fix:
- cli.py: add "docker_env": "TERMINAL_DOCKER_ENV" to env_mappings;
  serialise dict values with json.dumps alongside existing list path
- gateway/run.py: same additions to _terminal_env_map and serialisation
- hermes_cli/config.py: add "terminal.docker_env": "TERMINAL_DOCKER_ENV"
  to _config_to_env_sync so `hermes config set terminal.docker_env …`
  persists to .env correctly
- tools/terminal_tool.py: add docker_env key to _get_env_config() reading
  TERMINAL_DOCKER_ENV via _parse_env_var with default "{}"

Tests: add test_docker_env_is_bridged_everywhere to
tests/tools/test_terminal_config_env_sync.py — stash-verified: fails on
origin/main, passes with fix.

Fixes #20537

tests/tools/test_terminal_config_env_sync.py

@@ -208,3 +208,19 @@ def test_docker_mount_cwd_to_workspace_is_bridged_everywhere():
     assert "docker_mount_cwd_to_workspace" in _gateway_env_map_keys()
     assert "docker_mount_cwd_to_workspace" in _save_config_env_sync_keys()
     assert "TERMINAL_DOCKER_MOUNT_CWD_TO_WORKSPACE" in _terminal_tool_env_var_names()
+
+
+def test_docker_env_is_bridged_everywhere():
+    """Regression pin for docker_env config key being silently ignored.
+
+    ``terminal.docker_env`` in config.yaml specifies extra env vars to inject
+    into the Docker container at runtime.  The key was present in
+    _create_environment's container_config consumer (line ~1130) but never
+    bridged from config.yaml to TERMINAL_DOCKER_ENV, so the dict was always
+    empty regardless of what the user set.  Guard all four bridging points so
+    this cannot regress.
+    """
+    assert "docker_env" in _cli_env_map_keys()
+    assert "docker_env" in _gateway_env_map_keys()
+    assert "docker_env" in _save_config_env_sync_keys()
+    assert "TERMINAL_DOCKER_ENV" in _terminal_tool_env_var_names()