feat: migrate agents-access to org-scoped system role for proper chat RBAC (#24438)

The agents-access role previously granted chat permissions at user
scope, but chats are org-scoped objects. Rego skips user-level perms
when org_owner is set, making the grants invisible. Handler-level
band-aids used synthetic non-org-scoped objects as a workaround.

  - Migrates agents-access from users.rbac_roles (site-level) to
    organization_members.roles (org-scoped) via DB migration
  - Redefines agents-access as a predefined org-scoped builtin role
    alongside organization-admin, organization-auditor, etc., with
    Member permissions granting chat create/read/update
  - Excludes ResourceChat from OrgMemberPermissions so org membership
    alone no longer grants chat access
  - Fixes handler Authorize checks to use org-scoped objects with
semantically correct actions (ActionUpdate for message/tool operations)
  - Grants org admins the ability to assign agents-access

Closes #24250
Fixes CODAGT-174

Note: this does not update the "Usage" endpoints. Tracked by CODAGT-161.
> 🤖

coderd/database/dbauthz/setup_test.go

@@ -242,6 +242,7 @@ func (s *MethodTestSuite) SubtestWithDB(db database.Store, testCaseF func(db dat
 			slice.Contains([]string{
 				"GetAuthorizedWorkspaces",
 				"GetAuthorizedTemplates",
+				"GetDefaultChatModelConfig",
 			}, methodName) {
 			// Some methods do not make RBAC assertions because they use
 			// SQL. We still want to test that they return an error if the