Post-merge fixes for the email gateway (PR #797):
1. Add Platform.EMAIL to all 4 platform-to-toolset/config mapping
dicts in gateway/run.py. Without this, email sessions silently
fell back to the Telegram toolset because these dicts were added
after the PR branched off main.
2. Add email (and signal) to hermes_cli/tools_config.py and
hermes_cli/skills_config.py PLATFORMS dicts so they appear in
'hermes tools' and 'hermes skills' CLI commands.
3. Add full email setup documentation:
- website/docs/user-guide/messaging/email.md — setup guide with
Gmail/Outlook instructions, configuration, troubleshooting,
security advice, and env var reference
- Update messaging/index.md — add email to architecture diagram,
platform toolset table, security examples, and next steps
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