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/queries/siteconfig.sql

@@ -175,6 +175,14 @@ SELECT
 INSERT INTO site_configs (key, value) VALUES ('agents_chat_explore_model_override', $1)
 ON CONFLICT (key) DO UPDATE SET value = $1 WHERE site_configs.key = 'agents_chat_explore_model_override';
 
+-- name: GetChatGeneralModelOverride :one
+SELECT
+	COALESCE((SELECT value FROM site_configs WHERE key = 'agents_chat_general_model_override'), '') :: text AS model_config_id;
+
+-- name: UpsertChatGeneralModelOverride :exec
+INSERT INTO site_configs (key, value) VALUES ('agents_chat_general_model_override', $1)
+ON CONFLICT (key) DO UPDATE SET value = $1 WHERE site_configs.key = 'agents_chat_general_model_override';
+
 -- name: GetChatDesktopEnabled :one
 SELECT
 	COALESCE((SELECT value = 'true' FROM site_configs WHERE key = 'agents_desktop_enabled'), false) :: boolean AS enable_desktop;