Removing an env-seeded credential (e.g. from OPENROUTER_API_KEY) via
'hermes auth' previously had no lasting effect -- the entry was deleted
from auth.json but load_pool() re-created it on the next call because
the env var was still set.
Now auth_remove_command detects env-sourced entries (source starts with
'env:') and calls the new remove_env_value() to strip the var from both
.env and os.environ, preventing re-seeding.
Changes:
- hermes_cli/config.py: add remove_env_value() -- atomically removes a
line from .env and pops from os.environ
- hermes_cli/auth_commands.py: auth_remove_command clears env var when
removing an env-seeded pool entry
- 8 new tests covering remove_env_value and the full zombie-credential
lifecycle (remove -> reload -> stays gone)
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