fix(config): bridge docker_volumes and docker_forward_env in config set

[Dusk1e]

What & why

terminal.* settings are read by terminal_tool from TERMINAL_* env
vars, so config values must be bridged into the environment at three points:

1. cli.py → env_mappings
2. gateway/run.py → _terminal_env_map
3. hermes_cli/config.py:set_config_value → _config_to_env_sync

terminal.docker_volumes and terminal.docker_forward_env were wired into
the first two but missing from the third. As a result, running:

hermes config set terminal.docker_volumes '["/host/proj:/workspace"]'

updated config.yaml but left the live process's TERMINAL_DOCKER_VOLUMES
stale — the bind mounts (and forwarded env vars) silently did nothing until
a full restart. Users saw "I saved the setting, but it isn't applied."

This is the same drift class already fixed for docker_run_as_host_user
and docker_mount_cwd_to_workspace.

Fix

Add both keys to _config_to_env_sync in set_config_value. No new
serialization logic is introduced: these are JSON-valued keys that
terminal_tool parses via json.loads, and the existing str(value)
path already produces valid JSON — identical to how the neighboring
terminal.docker_env key is handled. save_env_value() updates
os.environ and invalidates the env cache, so the change takes effect in
the running process immediately.

Tests

• Added test_docker_volumes_is_bridged_everywhere and
  test_docker_forward_env_is_bridged_everywhere, pinning all four bridge
  points (the established pattern for this regression class).
• Updated the test_save_config_set_supports_critical_bridged_keys
  docstring, which previously listed these two keys as known gaps.

How to test

hermes config set terminal.docker_volumes '["/host/proj:/workspace"]'
# TERMINAL_DOCKER_VOLUMES is now set in the active process;
# the mount applies without a restart.

Regression suite:

scripts/run_tests.sh tests/tools/test_terminal_config_env_sync.py