Add scoped-model configuration to dashboard settings

[m-aebrer]

Summary

Add a dashboard Settings component for configuring the persistent scoped-model list used for model cycling. It should provide browser parity with the saved configuration from the TUI's /scoped-models selector while making ordering, persistence, and "all models" behavior explicit.

Current Behavior

The TUI /scoped-models selector lets users enable, disable, and reorder models, then persist the result as enabledModels in settings.

The dashboard currently:

• Shows the active runtime's scopedModels in the session model picker.
• Provides Settings controls for the default model and per-agent model fallbacks.
• Cannot display or edit persistent enabledModels.
• Cannot persist scoped models because get_settings and set_settings do not expose that field.

Users must edit settings.json or return to the TUI to configure model cycling.

Proposed Behavior

Add a Scoped models editor to dashboard Settings that:

• Loads all available models and the effective persisted enabledModels scope.
• Groups models by provider and supports search.
• Enables or disables individual models and whole providers.
• Shows enabled models in their model-cycling order.
• Allows enabled models to be reordered.
• Uses staged changes with an explicit save action.
• Persists canonical provider/model references through the settings RPC.
• Treats all models selected as an unfiltered scope and removes the persisted filter.
• Prevents saving an ambiguous zero-model scope and displays an explicit validation message.
• Explains that the setting applies to newly created sessions and does not mutate running sessions.
• Surfaces validation, persistence, and project-shadowing warnings verbatim.

Acceptance Criteria

• Dashboard Settings displays the persisted scoped-model configuration.
• Available models are searchable and grouped by provider.
• Users can enable or disable individual models.
• Users can enable or disable all models for a provider.
• Users can enable all models.
• Enabled models can be reordered, and the saved order determines model-cycling order.
• Saving a partial selection persists ordered canonical provider/model entries as enabledModels.
• Saving all available models clears the enabledModels filter rather than freezing the current model inventory.
• A zero-model selection may be used while editing but cannot be saved silently as "all models."
• Existing pattern-based enabledModels settings are resolved using the core model resolver rather than reimplementing pattern matching in the browser.
• The RPC contract has an explicit representation for clearing enabledModels; it does not rely on serializing undefined.
• Invalid input or failed writes leave the prior durable setting intact and surface an explicit error.
• A project-level enabledModels override that shadows a global dashboard write produces a visible warning.
• The UI works on desktop and mobile layouts.
• Tests cover RPC validation/persistence/clearing and dashboard selection, ordering, save, and error states.
• User-facing dashboard, settings, and RPC documentation is updated.

Context

Scoped-model UI was deliberately sequenced after the dashboard foundation work:

• Build first-class dreb web dashboard #307
• Design dreb dashboard UX/UI for TUI feature parity #311

The TUI already provides the reference behavior through /scoped-models.

Technical Notes

Relevant implementation areas include:

• packages/coding-agent/src/core/settings-manager.ts
  • Existing getEnabledModels() and setEnabledModels() methods.
• packages/coding-agent/src/core/model-resolver.ts
  • Reuse resolveModelScope() for existing patterns and ordering.
• packages/coding-agent/src/modes/rpc/rpc-types.ts
  • Add enabledModels to settings snapshot/update types.
  • Define an explicit clear representation, such as null in updates.
• packages/coding-agent/src/modes/rpc/rpc-mode.ts
  • Add snapshot mapping, atomic validation, persistence, and shadow warnings.
• packages/dashboard/src/shared/protocol.ts
  • Extend dashboard settings DTOs.
• packages/dashboard/src/client/screens/settings.tsx
  • Add the scoped-model editor, likely reusing the existing model-picker primitives.
• packages/dashboard/test/client/screens.test.tsx
  • Cover interaction and error behavior.

Documentation should remain synchronized across:

• README.md
• packages/coding-agent/README.md
• packages/coding-agent/docs/settings.md
• packages/coding-agent/docs/rpc.md
• packages/coding-agent/docs/dashboard.md
• packages/dashboard/README.md