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/db2sdk/db2sdk.go

@@ -1550,6 +1550,7 @@ func Chat(c database.Chat, diffStatus *database.ChatDiffStatus, files []database
 	}
 	chat := codersdk.Chat{
 		ID:                c.ID,
+		OrganizationID:    c.OrganizationID,
 		OwnerID:           c.OwnerID,
 		LastModelConfigID: c.LastModelConfigID,
 		Title:             c.Title,

coderd/database/db2sdk/db2sdk_test.go

@@ -530,6 +530,7 @@ func TestChat_AllFieldsPopulated(t *testing.T) {
 	input := database.Chat{
 		ID:                uuid.New(),
 		OwnerID:           uuid.New(),
+		OrganizationID:    uuid.New(),
 		WorkspaceID:       uuid.NullUUID{UUID: uuid.New(), Valid: true},
 		BuildID:           uuid.NullUUID{UUID: uuid.New(), Valid: true},
 		AgentID:           uuid.NullUUID{UUID: uuid.New(), Valid: true},