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, 30 insertions, 0 deletions

diff --git a/tests/tmux-util/fake-fzf b/tests/tmux-util/fake-fzf
new file mode 100755
index 0000000..476bf7f
--- /dev/null
+++ b/tests/tmux-util/fake-fzf
@@ -0,0 +1,30 @@
+#!/bin/sh
+# Fake fzf for testing tmux-util.
+#
+# Control via env vars:
+#   FAKE_FZF_CHOICE_LINE  — 1-based line index into stdin; that line is
+#                           printed as the selection and exit 0.
+#   FAKE_FZF_CHOICE       — print this literal string as the selection and
+#                           exit 0 (ignored if FAKE_FZF_CHOICE_LINE is set).
+#   (neither set)         — exit 130 (user cancelled).
+#
+# Every invocation is logged to $FAKE_TMUX_DIR/calls.log as one line.
+
+: "${FAKE_TMUX_DIR:?FAKE_TMUX_DIR must be set}"
+printf 'fzf %s\n' "$*" >> "$FAKE_TMUX_DIR/calls.log"
+
+if [ -n "${FAKE_FZF_CHOICE_LINE:-}" ]; then
+    awk -v n="$FAKE_FZF_CHOICE_LINE" 'NR == n { print; exit }'
+    exit 0
+fi
+
+if [ -n "${FAKE_FZF_CHOICE:-}" ]; then
+    # Drain stdin so the pipe upstream doesn't SIGPIPE
+    cat >/dev/null
+    printf '%s\n' "$FAKE_FZF_CHOICE"
+    exit 0
+fi
+
+# Cancelled
+cat >/dev/null
+exit 130