libtmux:set-environment

Set one raw tmux environment entry and return its COMMAND-OUTCOME. SCOPE is required: :GLOBAL rejects TARGET; :SESSION requires a captured session. NAME must match [A-Za-z_][A-Za-z0-9_]*. VALUE is literal unless FORMAT-P explicitly requests tmux format expansion. HIDDEN sets tmux's hidden flag. This mutates one raw table, not an effective merged child environment. Never retries or refreshes captured observations. See UNSET-ENVIRONMENT for deletion and REMOVE-ENVIRONMENT for an explicit tombstone.