Restructures the security section so the admin/user distinction is a
first-class concept rather than buried under 'Slash Command Access
Control'. The new section makes explicit that:
- Slash commands are the first capability gated by the tier split today
- Future gating (tools, model switching, etc.) will hang off the same
admin/user distinction, so configuring it now is forward-compatible
- Allowlists vs the admin/user split solve different problems and are
contrasted up front
Heading renamed: 'Slash Command Access Control' -> 'Admins vs Regular
Users'. The platform-specific pages (telegram.md, discord.md) keep the
old heading since slash gating IS the only thing they currently gate.
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