feat: add .zip document support and auto-mount cache dirs into remote backends (#4846)

- Add .zip to SUPPORTED_DOCUMENT_TYPES so gateway platforms (Telegram,
  Slack, Discord) cache uploaded zip files instead of rejecting them.
- Add get_cache_directory_mounts() and iter_cache_files() to
  credential_files.py for host-side cache directory passthrough
  (documents, images, audio, screenshots).
- Docker: bind-mount cache dirs read-only alongside credentials/skills.
  Changes are live (bind mount semantics).
- Modal: mount cache files at sandbox creation + resync before each
  command via _sync_files() with mtime+size change detection.
- Handles backward-compat with legacy dir names (document_cache,
  image_cache, audio_cache, browser_screenshots) via get_hermes_dir().
- Container paths always use the new cache/<subdir> layout regardless
  of host layout.

This replaces the need for a dedicated extract_archive tool (PR #4819)
— the agent can now use standard terminal commands (unzip, tar) on
uploaded files inside remote containers.

Closes: related to PR #4819 by kshitijk4poor

gateway/platforms/base.py

@@ -235,6 +235,7 @@ SUPPORTED_DOCUMENT_TYPES = {
     ".pdf": "application/pdf",
     ".md": "text/markdown",
     ".txt": "text/plain",
+    ".zip": "application/zip",
     ".docx": "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
     ".xlsx": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
     ".pptx": "application/vnd.openxmlformats-officedocument.presentationml.presentation",