fix(gateway): block .config, .gcloud, and macOS Keychains from media delivery

[nicha16]

Extends _MEDIA_DELIVERY_DENIED_HOME_SUBPATHS to deny:

• ~/.config/ (tokens, app credentials)
• ~/.gcloud/ (Google Cloud service account keys)
• ~/Library/Keychains/ (macOS keychain databases)

These paths are already blocked in strict mode but were missing from the default non-strict denylist.

[alt-glitch]

This was generated by AI during triage.

Heads-up for review: main already denies .gcloud and Library/Keychains in _MEDIA_DELIVERY_DENIED_HOME_SUBPATHS (gateway/platforms/base.py). This diff reintroduces the _MEDIA_DELIVERY_DENIED_* constants at an older location with a smaller set (branch appears based on stale main) and also bundles an unrelated change to tools/send_message_tool.py _get_cron_auto_delivery_target(). Recommend rebasing onto current main and isolating the denylist change from the cron-delivery change.

[teknium1]

Thanks for the security-focused contribution. The requested default-mode media-delivery protection is already present on current main; this is an automated hermes-sweeper review.

• gateway/platforms/base.py:1025-1035 already denies the broader ~/.config/ tree, ~/.gcloud/, and ~/Library/Keychains/.
• gateway/platforms/base.py:1327-1330 applies that denylist in the default non-strict delivery path.
• git blame attributes those entries to d682f320b35a13084371a541a835e1d988c982b8, which is contained in v2026.7.1.
• The PR also includes unrelated cron/send-message changes, while its denylist diff is superseded by the current implementation.

Closing as implemented on main.