fix(host): scope personal subscription access by actor

[touch2be]

Summary

• allow members permitted by the Host BYOS policy to connect and manage their own Codex/xAI subscriptions
• scope personal subscription metadata, credentials, refreshes, model visibility, execution, and disconnects to the authenticated actor
• keep shared Host model offerings on the Host credential namespace while direct personal selections use the member's credential
• expose personal subscription sign-in in provider settings only when the effective BYOS policy permits it

Security boundaries

• personal connection IDs are opaque and user-scoped
• members cannot list, use, refresh, disconnect, or mutate Host-global or another user's subscription
• model offering authorization stays actor-based, while trusted post-authorization routing explicitly selects Host credential execution for shared offerings
• denied members receive 403 for personal subscription management

Testing

• vp check
• pnpm run slop-check
• focused personal subscription and Codex routing suites
• full clean-environment suite: 212 files, 1112 tests passed