feat: add organization scoping to chats (#23827)

Fixes https://github.com/coder/internal/issues/1436

* Adds organization_id to chats with backfill (workspace org → user org membership → default org)
* No support yet for ACLs (follow-up issue)
- Cross-org workspace binding rejected (both in `CreateChatRequest` and in `create_workspace` tool
- Adds `OrganizationAutocomplete` to `AgentCreateForm`
- Docs updated with `organization_id` in chats-api.md

> 🤖 Written by a Coder Agent. Reviewed by many humans and many agents.

---------

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>

site/src/api/queries/chats.test.ts

@@ -80,6 +80,7 @@ const makeChat = (
 	overrides?: Partial<TypesGen.Chat>,
 ): TypesGen.Chat => ({
 	id,
+	organization_id: "test-org-id",
 	owner_id: "owner-1",
 	last_model_config_id: "model-1",
 	mcp_server_ids: [],

site/src/api/typesGenerated.ts

@@ -1186,6 +1186,7 @@ export interface ChangePasswordWithOneTimePasscodeRequest {
  */
 export interface Chat {
 	readonly id: string;
+	readonly organization_id: string;
 	readonly owner_id: string;
 	readonly workspace_id?: string;
 	readonly build_id?: string;
@@ -2488,6 +2489,7 @@ export interface CreateChatProviderConfigRequest {
  * CreateChatRequest is the request to create a new chat.
  */
 export interface CreateChatRequest {
+	readonly organization_id: string;
 	readonly content: readonly ChatInputPart[];
 	readonly system_prompt?: string;
 	readonly workspace_id?: string;