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/telemetry/telemetry_test.go

@@ -1646,6 +1646,7 @@ func TestChatsTelemetry(t *testing.T) {
 	})
 
 	rootChat, err := db.InsertChat(ctx, database.InsertChatParams{
+		OrganizationID:    org.ID,
 		OwnerID:           user.ID,
 		LastModelConfigID: modelCfg.ID,
 		Title:             "Root Chat",
@@ -1657,6 +1658,7 @@ func TestChatsTelemetry(t *testing.T) {
 
 	// Create a child chat (has parent + root).
 	childChat, err := db.InsertChat(ctx, database.InsertChatParams{
+		OrganizationID:    org.ID,
 		OwnerID:           user.ID,
 		LastModelConfigID: modelCfg2.ID,
 		Title:             "Child Chat",