feat: auto-assign agents-access role to new users when experiment enabled (#23968)

When the `agents` experiment is enabled, new users are automatically
granted the `agents-access` role at creation time so they can use Coder
Agents without manual admin intervention.

- Auto-assigns in `CreateUser()` — covers admin API, OAuth, and OIDC
creation paths
- Skips auto-assign for OIDC users when enterprise site role sync is
enabled (sync overwrites roles on every login; those admins should use
`--oidc-user-role-default` instead)
- CLI `create-admin-user` bypasses `CreateUser()` but creates `owner`
users who already have all permissions

> 🤖 Written by a Coder Agent. Will be reviewed by a human.

coderd/users.go

@@ -1619,6 +1619,18 @@ func (api *API) CreateUser(ctx context.Context, store database.Store, req Create
 		rbacRoles = req.RBACRoles
 	}
 
+	// When the agents experiment is enabled, auto-assign the
+	// agents-access role so new users can use Coder Agents
+	// without manual admin intervention. Skip this for OIDC
+	// users when site role sync is enabled, because the sync
+	// will overwrite roles on every login anyway — those
+	// admins should use --oidc-user-role-default instead.
+	if api.Experiments.Enabled(codersdk.ExperimentAgents) &&
+		!(req.LoginType == database.LoginTypeOIDC && api.IDPSync.SiteRoleSyncEnabled()) &&
+		!slices.Contains(rbacRoles, codersdk.RoleAgentsAccess) {
+		rbacRoles = append(rbacRoles, codersdk.RoleAgentsAccess)
+	}
+
 	var user database.User
 	err := store.InTx(func(tx database.Store) error {
 		orgRoles := make([]string, 0)