When terminal.backend is docker, inbound documents uploaded via messaging
platforms (Telegram, Slack, Discord, Feishu, Email, etc.) are cached at a host
path under ~/.hermes/cache/documents, but the container sandbox only sees them
at the auto-mounted /root/.hermes/cache/documents path.
This PR adds to_agent_visible_cache_path() in tools/credential_files.py (the
natural sibling to get_cache_directory_mounts()) and calls it at the
document-context-injection site in gateway/run.py so the agent always receives
a path it can open directly, matching the mount layout already established
by get_cache_directory_mounts() (#4846).
Scope: only Docker backend for now; other backends use different mount
semantics and are left unchanged until verified.
Fixes#18787
7c0766e06a