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/workspaceagents_active_chat_internal_test.go

@@ -35,6 +35,7 @@ func TestActiveAgentChatDefinitionsAgree(t *testing.T) {
 	for _, archived := range []bool{false, true} {
 		for _, status := range database.AllChatStatusValues() {
 			chat, err := db.InsertChat(ctx, database.InsertChatParams{
+				OrganizationID:    org.ID,
 				Status:            status,
 				OwnerID:           owner.ID,
 				LastModelConfigID: modelConfig.ID,