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/database/querier.go

@@ -308,6 +308,7 @@ type sqlcQuerier interface {
 	// loading file content.
 	GetChatFileMetadataByChatID(ctx context.Context, chatID uuid.UUID) ([]GetChatFileMetadataByChatIDRow, error)
 	GetChatFilesByIDs(ctx context.Context, ids []uuid.UUID) ([]ChatFile, error)
+	GetChatGeneralModelOverride(ctx context.Context) (string, error)
 	// GetChatIncludeDefaultSystemPrompt preserves the legacy default
 	// for deployments created before the explicit include-default toggle.
 	// When the toggle is unset, a non-empty custom prompt implies false;
@@ -1174,6 +1175,7 @@ type sqlcQuerier interface {
 	UpsertChatDiffStatus(ctx context.Context, arg UpsertChatDiffStatusParams) (ChatDiffStatus, error)
 	UpsertChatDiffStatusReference(ctx context.Context, arg UpsertChatDiffStatusReferenceParams) (ChatDiffStatus, error)
 	UpsertChatExploreModelOverride(ctx context.Context, value string) error
+	UpsertChatGeneralModelOverride(ctx context.Context, value string) error
 	UpsertChatIncludeDefaultSystemPrompt(ctx context.Context, includeDefaultSystemPrompt bool) error
 	UpsertChatPlanModeInstructions(ctx context.Context, value string) error
 	UpsertChatRetentionDays(ctx context.Context, retentionDays int32) error