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/rbac/authz.go

@@ -689,11 +689,11 @@ func ConfigWithoutACL() regosql.ConvertConfig {
 }
 
 // ConfigChats is the configuration for converting rego to SQL when
-// the target table is "chats", which has no organization_id or ACL
+// the target table is "chats", which has no ACL
 // columns.
 func ConfigChats() regosql.ConvertConfig {
 	return regosql.ConvertConfig{
-		VariableConverter: regosql.ChatConverter(),
+		VariableConverter: regosql.NoACLConverter(),
 	}
 }