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/dump.sql

@@ -1431,7 +1431,8 @@ CREATE TABLE chats (
     pin_order integer DEFAULT 0 NOT NULL,
     last_read_message_id bigint,
     last_injected_context jsonb,
-    dynamic_tools jsonb
+    dynamic_tools jsonb,
+    organization_id uuid NOT NULL
 );
 
 CREATE TABLE connection_logs (
@@ -3789,6 +3790,8 @@ CREATE INDEX idx_chats_labels ON chats USING gin (labels);
 
 CREATE INDEX idx_chats_last_model_config_id ON chats USING btree (last_model_config_id);
 
+CREATE INDEX idx_chats_organization_id ON chats USING btree (organization_id);
+
 CREATE INDEX idx_chats_owner ON chats USING btree (owner_id);
 
 CREATE INDEX idx_chats_parent_chat_id ON chats USING btree (parent_chat_id);
@@ -4104,6 +4107,9 @@ ALTER TABLE ONLY chats
 ALTER TABLE ONLY chats
     ADD CONSTRAINT chats_last_model_config_id_fkey FOREIGN KEY (last_model_config_id) REFERENCES chat_model_configs(id);
 
+ALTER TABLE ONLY chats
+    ADD CONSTRAINT chats_organization_id_fkey FOREIGN KEY (organization_id) REFERENCES organizations(id) ON DELETE CASCADE;
+
 ALTER TABLE ONLY chats
     ADD CONSTRAINT chats_owner_id_fkey FOREIGN KEY (owner_id) REFERENCES users(id) ON DELETE CASCADE;