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

hermes_cli/config.py

@@ -4843,6 +4843,7 @@ def set_config_value(key: str, value: str):
         "terminal.vercel_runtime": "TERMINAL_VERCEL_RUNTIME",
         "terminal.docker_mount_cwd_to_workspace": "TERMINAL_DOCKER_MOUNT_CWD_TO_WORKSPACE",
         "terminal.docker_run_as_host_user": "TERMINAL_DOCKER_RUN_AS_HOST_USER",
+        "terminal.docker_env": "TERMINAL_DOCKER_ENV",
         # terminal.cwd intentionally excluded — CLI resolves at runtime,
         # gateway bridges it in gateway/run.py. Persisting to .env causes
         # stale values to poison child processes.