feat: add general subagent model override (#24610)

Adds a deployment-wide admin override for general delegated subagents.

## What changed
- store the general override in `site_configs` and expose it through the
shared `agent-model-override/{context}` API
- apply the general override when spawning delegated general subagents,
while preserving the existing Explore override behavior
- reuse a shared Agents settings form for the general and Explore
override sections

## Validation
- `make gen`
- `go test ./coderd -run 'TestChatModelOverrides'`
- `go test ./coderd/x/chatd -run
'TestSpawnAgent_(GeneralUsesConfiguredModelOverride|GeneralOverrideLogsAndFallsBackWhenCredentialsUnavailable|GeneralOverrideLogsAndFallsBackWhenProviderDisabled)'`
- `pnpm -C site lint:types`
- `pnpm -C site test:storybook --
AgentSettingsAgentsPageView.stories.tsx`
- `make lint`
- `make pre-commit`

> Mux is acting on Mike's behalf.

coderd/coderd.go

@@ -1184,8 +1184,8 @@ func New(options *Options) *API {
 				r.Put("/system-prompt", api.putChatSystemPrompt)
 				r.Get("/plan-mode-instructions", api.getChatPlanModeInstructions)
 				r.Put("/plan-mode-instructions", api.putChatPlanModeInstructions)
-				r.Get("/explore-model-override", api.getChatExploreModelOverride)
-				r.Put("/explore-model-override", api.putChatExploreModelOverride)
+				r.Get("/agent-model-override/{context}", api.getChatAgentModelOverride)
+				r.Put("/agent-model-override/{context}", api.putChatAgentModelOverride)
 				r.Get("/desktop-enabled", api.getChatDesktopEnabled)
 				r.Put("/desktop-enabled", api.putChatDesktopEnabled)
 				r.Get("/debug-logging", api.getChatDebugLogging)