feat(agents): refine Cursor SDK runtime setup

[SDSLeon]

Summary

• Add a first-class Cursor SDK runtime setup, including encrypted API-key storage, global package discovery/update support, model catalog loading, and persisted runtime selection.
• Refine provider model defaults and controls, including safe default-model visibility and explicit Fast/Thinking preference persistence.
• Improve provider setup/status handling across environments and keep shared runtime/config payloads provider-agnostic.
• Harden Cursor SDK model selection so only catalog-advertised model IDs are launched, and expand regression coverage around setup, loading, models, secrets, drafts, and updates.

Motivation

Cursor can run through either ACP or its SDK, but setup, credentials, model selection, and runtime availability need to remain clear and reliable across local and remote environments. This consolidates those paths and removes invalid fallback behavior that could launch an SDK session with a model ID the installed SDK did not advertise.

Testing

• pnpm run typecheck
• pnpm run lint
• pnpm run fmt:check
• pnpm run test — 694 files passed, 3 skipped; 7,384 tests passed, 67 skipped
• pnpm i18n:extract — 2,719 messages, 0 missing in all 12 non-English catalogs
• Not run; reason:

Screenshots

Not captured. The settings/runtime flows and state transitions are covered by renderer and supervisor regression tests.

Linked issue

None.