feat: mount skill credential files + fix env passthrough for remote backends (#3671)

Two related fixes for remote terminal backends (Modal/Docker):

1. NEW: Credential file mounting system
   Skills declare required_credential_files in frontmatter. Files are
   mounted into Docker (read-only bind mounts) and Modal (mounts at
   creation + sync via exec on each command for mid-session changes).
   Google Workspace skill updated with the new field.

2. FIX: Docker backend now includes env_passthrough vars
   Skills that declare required_environment_variables (e.g. Notion with
   NOTION_API_KEY) register vars in the env_passthrough system. The
   local backend checked this, but Docker's forward_env was a separate
   disconnected list. Now Docker exec merges both sources, so
   skill-declared env vars are forwarded into containers automatically.

   This fixes the reported issue where NOTION_API_KEY in ~/.hermes/.env
   wasn't reaching the Docker container despite being registered via
   the Notion skill's prerequisites.

Closes #3665

skills/productivity/google-workspace/SKILL.md

@@ -4,6 +4,11 @@ description: Gmail, Calendar, Drive, Contacts, Sheets, and Docs integration via
 version: 1.0.0
 author: Nous Research
 license: MIT
+required_credential_files:
+  - path: google_token.json
+    description: Google OAuth2 token (created by setup script)
+  - path: google_client_secret.json
+    description: Google OAuth2 client credentials (downloaded from Google Cloud Console)
 metadata:
   hermes:
     tags: [Google, Gmail, Calendar, Drive, Sheets, Docs, Contacts, Email, OAuth]