fix(tmux): scope numeric session window creation

[kfu95]

Summary

• target tmux sessions as session: when checking for duplicate windows and creating task windows
• fixes purely numeric session names being interpreted ambiguously by tmux new-window -t
• adds fake-tmux command-shape coverage and a real isolated tmux numeric-session smoke regression

Validation

• bash tests/fm-backend.test.sh
• bash tests/fm-backend-tmux-smoke.test.sh
• shellcheck bin/.sh bin/backends/.sh tests/*.sh

Note

• no-mistakes cannot publish directly from this account: upstream push to kunchenguid/firstmate returns 403 for kfu95, so this PR is from fork kfu95/firstmate.