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.

1 files changed, 21 insertions, 0 deletions

diff --git a/tests/tmux-util/fake-tmux b/tests/tmux-util/fake-tmux
index ba2d5cc..163ea24 100755
--- a/tests/tmux-util/fake-tmux
+++ b/tests/tmux-util/fake-tmux
@@ -97,6 +97,8 @@ case "$cmd" in
                 *)  shift ;;
             esac
         done
+        # tmux accepts a `=name` form to force exact match; strip the prefix.
+        session="${session#=}"
         while IFS=' ' read -r name attached pids _rest; do
             if [ "$name" = "$session" ]; then
                 exit 0
@@ -104,6 +106,25 @@ case "$cmd" in
         done < "$STATE"
         exit 1
         ;;
+    new-session)
+        detached=0
+        name=""
+        cwd=""
+        while [ "$#" -gt 0 ]; do
+            case "$1" in
+                -s) shift; name="$1"; shift ;;
+                -c) shift; cwd="$1"; shift ;;
+                -d) detached=1; shift ;;
+                *)  shift ;;
+            esac
+        done
+        attached=1
+        [ "$detached" -eq 1 ] && attached=0
+        printf '%s %s - 0 1 %s\n' "$name" "$attached" "${cwd:-/tmp}" >> "$STATE"
+        ;;
+    attach-session|switch-client)
+        # No state mutation needed — the call log already records intent.
+        ;;
     kill-session)
         session=""
         while [ "$#" -gt 0 ]; do