feat: add personal chat model overrides (#24715)

coderd/database/queries/siteconfig.sql

@@ -259,6 +259,30 @@ SET value = CASE
 END
 WHERE site_configs.key = 'agents_chat_debug_logging_allow_users';
 
+-- GetChatPersonalModelOverridesEnabled returns whether users may configure
+-- personal chat model overrides. It defaults to false when unset.
+-- name: GetChatPersonalModelOverridesEnabled :one
+SELECT
+	COALESCE((SELECT value = 'true' FROM site_configs WHERE key = 'agents_chat_personal_model_overrides_enabled'), false) :: boolean AS enabled;
+
+-- UpsertChatPersonalModelOverridesEnabled updates whether users may configure
+-- personal chat model overrides.
+-- name: UpsertChatPersonalModelOverridesEnabled :exec
+INSERT INTO site_configs (key, value)
+VALUES (
+    'agents_chat_personal_model_overrides_enabled',
+    CASE
+        WHEN sqlc.arg(enabled)::bool THEN 'true'
+        ELSE 'false'
+    END
+)
+ON CONFLICT (key) DO UPDATE
+SET value = CASE
+    WHEN sqlc.arg(enabled)::bool THEN 'true'
+    ELSE 'false'
+END
+WHERE site_configs.key = 'agents_chat_personal_model_overrides_enabled';
+
 -- GetChatTemplateAllowlist returns the JSON-encoded template allowlist.
 -- Returns an empty string when no allowlist has been configured (all templates allowed).
 -- name: GetChatTemplateAllowlist :one