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>

docs/ai-coder/agents/chats-api.md

@@ -26,6 +26,7 @@ curl -X POST https://coder.example.com/api/experimental/chats \
   -H "Coder-Session-Token: $CODER_SESSION_TOKEN" \
   -H "Content-Type: application/json" \
   -d '{
+    "organization_id": "<your-org-id>",
     "content": [
       {"type": "text", "text": "hello world"}
     ]
@@ -37,6 +38,7 @@ The response is the newly created `Chat` object:
 ```json
 {
   "id": "a1b2c3d4-...",
+  "organization_id": "...",
   "owner_id": "...",
   "workspace_id": null,
   "build_id": null,
@@ -82,6 +84,7 @@ A typical integration follows three steps:
 | Field             | Type                | Required | Description                                     |
 |-------------------|---------------------|----------|-------------------------------------------------|
 | `content`         | `ChatInputPart[]`   | yes      | The user's prompt as one or more content parts. |
+| `organization_id` | `uuid`              | yes      | The organization this chat belongs to.          |
 | `workspace_id`    | `uuid`              | no       | Pin the chat to a specific workspace.           |
 | `model_config_id` | `uuid`              | no       | Override the default model configuration.       |
 | `mcp_server_ids`  | `uuid[]`            | no       | Attach MCP servers to this chat.                |