Clarify tclone setup and preserve tmux attach env

[yiying-zhang]

Summary

• preserve GENSEE_TMP_ROOT, TMPDIR, and CONTAINERS_STORAGE_CONF when tmux attach panes re-enter gensee run attach
• document the host tclone setup in the README, including btrfs Podman storage, image-store matching, temp-root placement, and shared wrapper usage
• expand tclone docs with the setup and troubleshooting details found while debugging test-gensee

Why

Forks could be created successfully while the tmux pane failed to appear because the attach pane lost the storage/tmp environment and looked in a different Podman store. The README also did not contain enough information to reproduce the working test-gensee setup without outside notes.

Validation

• cargo fmt -p gensee-crate-cli
• cargo test -p gensee-crate-cli tclone_host_tmux_attach_command -- --nocapture
• git diff --check README.md

Companion docs PR: GenseeAI/os4agent#4.

[shengqi-gensee]

LGTM