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.

2 files changed, 101 insertions, 3 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
diff --git a/tests/tmux-util/test_tmux_util.py b/tests/tmux-util/test_tmux_util.py
index e313af6..afc1ebc 100644
--- a/tests/tmux-util/test_tmux_util.py
+++ b/tests/tmux-util/test_tmux_util.py
@@ -33,7 +33,7 @@ class TmuxUtilHarness(unittest.TestCase):
         # bin dir with the fakes symlinked under their canonical names
         self.bin_dir = os.path.join(self.tmp, "bin")
         os.makedirs(self.bin_dir)
-        for name in ("tmux", "kill", "sleep"):
+        for name in ("tmux", "kill", "sleep", "fzf"):
             fake = os.path.join(FAKES_DIR, f"fake-{name}")
             os.chmod(fake, os.stat(fake).st_mode | stat.S_IEXEC)
             os.symlink(fake, os.path.join(self.bin_dir, name))
@@ -128,8 +128,8 @@ class TestDispatch(TmuxUtilHarness):
         self.assertIn("Usage: tmux-util", result.stderr)
 
     def test_unimplemented_subcommand_exits_nonzero(self):
-        # go/pick/find/rename stub out for now; pick one to confirm the stub path.
-        result = self.run_script("pick")
+        # rename stubs out for now; pick one to confirm the stub path.
+        result = self.run_script("rename")
         self.assertNotEqual(result.returncode, 0)
         self.assertIn("not implemented yet", result.stderr)
 
@@ -549,5 +549,73 @@ class TestFindBoundary(TmuxUtilHarness):
         self.assertIn("missing pattern", result.stderr)
 
 
+# -----------------------------------------------------------------------------
+# pick — Normal cases
+# -----------------------------------------------------------------------------
+
+class TestPickNormal(TmuxUtilHarness):
+
+    def test_pick_attaches_chosen_session_outside_tmux(self):
+        self.set_sessions([
+            ("foo", 1, ["101:zsh"], 950, 1, "/tmp/foo"),
+            ("bar", 0, ["201:zsh"], 900, 1, "/tmp/bar"),
+        ])
+        # Pick the second line (bar)
+        env = {k: v for k, v in os.environ.items() if k != "TMUX"}
+        result = subprocess.run(
+            [SCRIPT, "pick"],
+            env={**env, "PATH": self.bin_dir + os.pathsep + env.get("PATH", ""),
+                 "FAKE_TMUX_DIR": self.tmp, "FAKE_FZF_CHOICE_LINE": "2"},
+            capture_output=True, text=True, timeout=10,
+        )
+        self.assertEqual(result.returncode, 0, msg=result.stderr)
+        calls = self.tmux_calls()
+        self.assertTrue(any("attach-session -t bar" in c for c in calls),
+                        f"expected attach-session -t bar in {calls!r}")
+
+    def test_pick_switches_client_inside_tmux(self):
+        self.set_sessions([
+            ("foo", 1, ["101:zsh"], 950, 1, "/tmp/foo"),
+            ("bar", 0, ["201:zsh"], 900, 1, "/tmp/bar"),
+        ])
+        result = self.run_script("pick", env_extra={
+            "TMUX": "/tmp/fake-tmux-socket,1234,0",
+            "FAKE_FZF_CHOICE_LINE": "1",  # first line → foo
+        })
+        self.assertEqual(result.returncode, 0, msg=result.stderr)
+        calls = self.tmux_calls()
+        self.assertTrue(any("switch-client -t foo" in c for c in calls),
+                        f"expected switch-client -t foo in {calls!r}")
+        self.assertFalse(any("attach-session" in c for c in calls),
+                         f"unexpected attach-session in {calls!r}")
+
+
+# -----------------------------------------------------------------------------
+# pick — Boundary cases
+# -----------------------------------------------------------------------------
+
+class TestPickBoundary(TmuxUtilHarness):
+
+    def test_pick_no_sessions_prints_message(self):
+        self.set_sessions([])
+        result = self.run_script("pick")
+        self.assertEqual(result.returncode, 0)
+        self.assertIn("No tmux sessions", result.stdout)
+        # fzf should never have been invoked
+        self.assertFalse(any(c.startswith("fzf ") for c in self.tmux_calls()),
+                         f"unexpected fzf call in {self.tmux_calls()!r}")
+
+    def test_pick_user_cancels_fzf_returns_zero_no_attach(self):
+        self.set_sessions([
+            ("foo", 1, ["101:zsh"], 950, 1, "/tmp/foo"),
+        ])
+        # No FAKE_FZF_CHOICE / FAKE_FZF_CHOICE_LINE → fzf exits 130 (cancelled)
+        result = self.run_script("pick")
+        self.assertEqual(result.returncode, 0, msg=result.stderr)
+        calls = self.tmux_calls()
+        self.assertFalse(any("attach-session" in c or "switch-client" in c for c in calls),
+                         f"unexpected attach/switch in {calls!r}")
+
+
 if __name__ == "__main__":
     unittest.main()