feat(tmux-util): add pick subcommand (fzf session switcher)

tmux-util pick lists every session ("attached"/"detached" plus name), pipes it through fzf, and attaches or switch-clients to the chosen one (matching the go subcommand's inside-vs-outside-tmux discipline).

If the user cancels fzf (Ctrl-C, Esc, empty selection), the pipeline returns empty and pick exits 0 without touching tmux state.

The new fake-fzf testing fake is driven by env vars:
- FAKE_FZF_CHOICE_LINE=<N>: return the Nth line of stdin as the selection
- FAKE_FZF_CHOICE=<string>: return the literal string (ignores stdin)
- (neither): exit 130 to simulate cancel

4 new tests cover Normal cases (pick second line, inside/outside tmux behavior) and Boundary cases (no sessions, user cancellation). Total suite: 43 tests, all green.

1 files changed, 38 insertions, 1 deletions

diff --git a/dotfiles/common/.local/bin/tmux-util b/dotfiles/common/.local/bin/tmux-util
index 5e0ce85..4669b03 100755
--- a/dotfiles/common/.local/bin/tmux-util
+++ b/dotfiles/common/.local/bin/tmux-util
@@ -36,6 +36,40 @@ EOF
 }
 
 # -----------------------------------------------------------------------------
+# pick
+# -----------------------------------------------------------------------------
+
+cmd_pick() {
+    local sessions
+    sessions=$(tmux list-sessions \
+        -F '#{session_attached} #{session_name}' 2>/dev/null || true)
+    if [ -z "$sessions" ]; then
+        echo "No tmux sessions to pick from."
+        return 0
+    fi
+
+    # Format for fzf: "<state>  <name>" — name is the second field of the
+    # selection line, which we recover after fzf returns.
+    local choice
+    choice=$(echo "$sessions" | awk '{
+        state = ($1 == "1") ? "attached" : "detached"
+        printf "%-9s %s\n", state, $2
+    }' | fzf --header="Pick a tmux session" | awk '{print $2}')
+
+    if [ -z "$choice" ]; then
+        # User cancelled (Ctrl-C or Esc); fzf exits 130, the pipeline
+        # returns empty.
+        return 0
+    fi
+
+    if [ -n "${TMUX:-}" ]; then
+        tmux switch-client -t "$choice"
+    else
+        tmux attach-session -t "$choice"
+    fi
+}
+
+# -----------------------------------------------------------------------------
 # find
 # -----------------------------------------------------------------------------
 
@@ -204,7 +238,10 @@ main() {
         find)
             cmd_find "$@"
             ;;
-        pick|rename)
+        pick)
+            cmd_pick "$@"
+            ;;
+        rename)
             echo "tmux-util: subcommand '$sub' is not implemented yet" >&2
             return 2
             ;;