Preserve ACP model descriptions in the catalog and pickers

[bradhallett]

What was wrong

ACP agents send a per-model description on config-option select options, and the wire schema already carries it through — but buildModelCatalogFromConfigOptions hard-coded description: "", and the desktop option mapping dropped the field even when present. When two advertised models share a display name (e.g. two providers both surfacing "GLM 4.7"), the desktop picker rendered two indistinguishable rows: the underlying provider/model values differ but were invisible anywhere in the UI. Mobile already maps and renders the field; it showed nothing only because the bridge sent "". Fixes #2062

What changed

• plugins/provider-acp/src/wire.ts — acpConfigOptionSelectOptionSchema now types description (acpOptionalString, agent JSON null normalized to undefined) instead of passing it through untyped.
• plugins/provider-acp/src/bridge/model-catalog.ts — the config-options path maps option.description ?? "", mirroring the session-models path a few lines below.
• apps/app/src/hooks/useThreadCreationOptions.ts, apps/app/src/components/pickers/ModelReasoningPicker.tsx — non-empty descriptions flow into picker options and render as a subtle second line in model menu rows (main list and More Models submenu), matching the mobile picker's subtitle pattern. Labels, tags, routeProviderId qualifiers, and the selected provider/model value are unchanged.
• apps/mobile — no code change; added a regression test pinning the existing mapping.

Wire/protocol: no HOST_DAEMON_PROTOCOL_VERSION bump. AvailableModel.description is already a required z.string() in the host-daemon contract (commands.ts), so this only populates an existing field; messages stay schema-valid for peers running older code. No CLI surface changes — bb provider models already prints raw ids. Matches the fix proposed in the issue.

How you verified

• New tests that fail before and pass after (verified by reverting the guarded line and watching each go red, then restoring):
  • plugins/provider-acp/src/bridge/model-catalog.test.ts — "keeps per-option descriptions that disambiguate duplicate model names".
  • apps/app/src/components/pickers/ModelReasoningPicker.test.tsx — "distinguishes duplicate preview models by description and preserves value" (also asserts selection emits the exact provider/model value).
  • apps/mobile/src/data/compose/execution-options.test.ts — "carries the model description verbatim into the picker option".
  • plugins/provider-acp/src/wire.test.ts — null→undefined normalization of the new field.
• pnpm exec turbo run typecheck --filter=bb-plugin-provider-acp --filter=@bb/app --filter=@bb/mobile — pass.
• pnpm exec turbo run test --filter=bb-plugin-provider-acp --filter=@bb/app --filter=@bb/mobile — pass (incl. full @bb/app suite).
• pnpm exec turbo run lint --filter=bb-plugin-provider-acp --filter=@bb/app --filter=@bb/mobile — pass.

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