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>

coderd/database/modelqueries.go

@@ -799,6 +799,7 @@ func (q *sqlQuerier) GetAuthorizedChats(ctx context.Context, arg GetChatsParams,
 			&i.Chat.LastReadMessageID,
 			&i.Chat.LastInjectedContext,
 			&i.Chat.DynamicTools,
+			&i.Chat.OrganizationID,
 			&i.HasUnread); err != nil {
 			return nil, err
 		}