feat: enable/disable providers from settings

Adds a per-provider toggle in Settings → Providers, persisted via daemon
config, so users can turn off providers they don't have installed (or
don't want to use) without removing them from the registry.

Disabled providers stay visible everywhere — settings, `paseo provider
ls`, MCP `list_providers`, model/mode lists — marked as disabled rather
than silently filtered. Availability probes are skipped for them, and
attempts to launch an agent or fetch models/modes from a disabled
provider return a clear "Provider '<id>' is disabled" error.

Schema additions (`enabled` on `ProviderSnapshotEntry`, MCP provider
summary) are backward-compatible: optional with a default of true so
older clients continue to parse new daemon messages.

CLI: `paseo provider ls` gains an ENABLED column; JSON output includes
the new field.

packages/server/src/shared/messages.ts

@@ -98,7 +98,6 @@ import type {
   AgentPermissionRequest,
   AgentPermissionResponse,
   AgentPersistenceHandle,
-  ProviderSnapshotEntry,
   ProviderStatus,
   AgentRuntimeInfo,
   AgentTimelineItem,
@@ -169,9 +168,10 @@ const AgentModelDefinitionSchema: z.ZodType<AgentModelDefinition> = z.object({
   defaultThinkingOptionId: z.string().optional(),
 });
 
-const ProviderSnapshotEntrySchema: z.ZodType<ProviderSnapshotEntry> = z.object({
+export const ProviderSnapshotEntrySchema = z.object({
   provider: AgentProviderSchema,
   status: ProviderStatusSchema,
+  enabled: z.boolean().optional().default(true),
   error: z.string().optional(),
   models: z.array(AgentModelDefinitionSchema).optional(),
   modes: z.array(AgentModeSchema).optional(),