Function is_global_cmd_env_injectable 

fn is_global_cmd_env_injectable(key: &str) -> bool

Whether key may be injected into a pane’s actual OS environment from the global settings’ cmd_env defaults (cmd_env_overrides, lowest priority — the block’s own cmd:env override, highest priority, is never filtered by this and can always set either key explicitly). Identity keys are excluded (reagentx P1, round 2 on #2694): without this, a global cmd_env.AGENTMUX_AGENT_ID setting would still land in every pane’s real environment even though resolve_agent_id_for_jekt never reads it for registration — shell integration echoes an injected AGENTMUX_AGENT_ID back via OSC 16162, and the frontend’s own POST /agentmux/reactive/register path (triggered by that OSC) would pick it up and register the pane under the shared value anyway, silently reopening the same-host identity collision this PR removes from the backend auto-register path specifically.