feat: unify CLI and TUI provider cards

[atomantic]

Summary

Matching CLI/TUI providers now appear as one AI Providers card with shared enablement and model catalogs, explicit CLI/TUI system-default buttons, mode-specific editors, and a Shell link that always launches TUI. Direct API connections and differently configured backends retain separate cards.

An input-gated migration enables both modes when either was enabled and unions their catalogs. Both execution IDs remain available to model selectors and saved tasks; existing arguments, mode defaults, consent, and the active mode survive upgrades. Catalog updates that deliberately repick defaults also repair a removed sibling default. Broader harness/backend connection design is tracked in #6359.

Test plan

• 108 client tests passed: AI Providers interactions and ProviderModelSelector mode selection.
• 158 server/migration tests passed (one platform skip): upgrade preservation/idempotence, shared updates, API response compatibility, TUI launch, batch refresh, caching, and seed parity.
• Client production build passed.
• Optional Claude review ran with enforced read-only tools. Its output was inconclusive due to malformed verdict formatting; its substantive finding was independently verified, fixed, and tested within the configured one-round cap.