feat(tmux-util): add go subcommand (attach-or-create)

tmux-util go <name> attaches to a session named <name> if it exists, creates it otherwise. Behavior depends on whether the caller is already inside tmux:

- Outside tmux: `tmux attach-session -t <name>` (existing) or `tmux new-session -s <name> -c $PWD` (new).
- Inside tmux (TMUX env set): `tmux switch-client -t <name>` (existing) or `tmux new-session -d -s <name> -c $PWD` followed by `switch-client` (new). Attaching from inside tmux would nest sessions and break the outer view, so the inside path uses switch-client instead.

The existence check uses `tmux has-session -t =<name>` with the leading `=` to force exact-match. Without it, tmux does prefix matching, which would let `go foo` resolve to a session named `foobar`.

I added 6 new tests covering both inside/outside-tmux paths, both create/attach paths, plus error handling for missing or empty name arguments. fake-tmux picked up handlers for new-session (mutates state), attach-session and switch-client (record-only), and the `=`-prefix form of has-session. Total suite: 32 tests, all green.

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