Drive TUI service-tier commands from model metadata

[aibrahim-oai]

Summary

• Build service-tier slash commands from the active model serviceTiers metadata instead of hardcoded command variants.
• Use the normalized tier name as the command token and dispatch or persist the backend tier id.
• Preserve the old tier-command UX generically: bare toggle plus on, off, and status.
• Refresh available dynamic commands when model/session metadata changes.
• Render selected service-tier status generically in status line, title, and related tooltips.

Verification

• Not run locally per repo guidance; CI will validate.

[aibrahim-oai]

This change is part of the following stack:

• Use structured service tiers across core and app-server #20822
  • Drive TUI service-tier commands from model metadata #20824 ◀

_{Change managed by git-spice.}

[chatgpt-codex-connector]

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