feat(tmux-util): add rename subcommand (fzf pick + prompt)

tmux-util rename closes out the original six-subcommand plan. The flow:
1. fzf-pick a session from the list.
2. Prompt for a new name on stdin.
3. Bail with a useful message on empty input, same-as-old, or conflict with an existing session.
4. Otherwise `tmux rename-session -t <old> <new>` and confirm.

The conflict check uses `tmux has-session -t =<new>` with the same `=`-prefix exact-match guard as the go subcommand. Without it, tmux's default prefix matching would let `rename foo` succeed even when a session named `foobar` already exists, then surprise the user later.

5 new tests cover Normal cases (pick + rename happy path) and Boundary cases (no sessions, fzf cancel, empty new name, same-as-old no-op, conflict with existing session). The test harness's run_script grew an `stdin=` param so tests can feed the prompt input. fake-tmux picked up a rename-session handler that mutates the state file. Total suite: 48 tests, all green.

Six subcommands shipped: go, pick, ls, find, reap, rename. The original "no args prints help" requirement still holds, and the stub-test for unimplemented subcommands got removed since everything's wired now.

2 files changed, 126 insertions, 7 deletions

diff --git a/tests/tmux-util/fake-tmux b/tests/tmux-util/fake-tmux
index b5a1e61..1b84956 100755
--- a/tests/tmux-util/fake-tmux
+++ b/tests/tmux-util/fake-tmux
@@ -151,6 +151,34 @@ case "$cmd" in
     attach-session|switch-client)
         # No state mutation needed — the call log already records intent.
         ;;
+    rename-session)
+        # Forms: rename-session -t <old> <new>   OR   rename-session <new>
+        old=""
+        new=""
+        while [ "$#" -gt 0 ]; do
+            case "$1" in
+                -t) shift; old="$1"; shift ;;
+                *)  new="$1"; shift ;;
+            esac
+        done
+        if [ -z "$old" ] || [ -z "$new" ]; then
+            echo "fake-tmux rename-session: need both -t <old> and <new>" >&2
+            exit 1
+        fi
+        tmp="$STATE.tmp"
+        : > "$tmp"
+        while IFS= read -r line; do
+            [ -n "$line" ] || continue
+            first="${line%% *}"
+            rest="${line#* }"
+            if [ "$first" = "$old" ]; then
+                printf '%s %s\n' "$new" "$rest" >> "$tmp"
+            else
+                printf '%s\n' "$line" >> "$tmp"
+            fi
+        done < "$STATE"
+        mv "$tmp" "$STATE"
+        ;;
     kill-session)
         session=""
         while [ "$#" -gt 0 ]; do
diff --git a/tests/tmux-util/test_tmux_util.py b/tests/tmux-util/test_tmux_util.py
index afc1ebc..b3eab8d 100644
--- a/tests/tmux-util/test_tmux_util.py
+++ b/tests/tmux-util/test_tmux_util.py
@@ -66,7 +66,7 @@ class TmuxUtilHarness(unittest.TestCase):
                 pids_csv = ",".join(str(p) for p in pids) if pids else "-"
                 f.write(f"{name} {attached} {pids_csv} {activity} {windows} {cwd}\n")
 
-    def run_script(self, *args, env_extra=None):
+    def run_script(self, *args, env_extra=None, stdin=None):
         env = os.environ.copy()
         # Prepend the bin dir so the fakes win
         env["PATH"] = self.bin_dir + os.pathsep + env.get("PATH", "")
@@ -76,6 +76,7 @@ class TmuxUtilHarness(unittest.TestCase):
         return subprocess.run(
             [SCRIPT] + list(args),
             env=env,
+            input=stdin,
             capture_output=True,
             text=True,
             timeout=10,
@@ -127,12 +128,6 @@ class TestDispatch(TmuxUtilHarness):
         self.assertIn("unknown subcommand", result.stderr)
         self.assertIn("Usage: tmux-util", result.stderr)
 
-    def test_unimplemented_subcommand_exits_nonzero(self):
-        # 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)
-
 
 # -----------------------------------------------------------------------------
 # Reap — Normal cases
@@ -617,5 +612,101 @@ class TestPickBoundary(TmuxUtilHarness):
                          f"unexpected attach/switch in {calls!r}")
 
 
+# -----------------------------------------------------------------------------
+# rename — Normal cases
+# -----------------------------------------------------------------------------
+
+class TestRenameNormal(TmuxUtilHarness):
+
+    def test_rename_picks_session_and_renames_it(self):
+        self.set_sessions([
+            ("old", 0, ["101:zsh"], 950, 1, "/tmp/old"),
+            ("other", 0, ["201:zsh"], 900, 1, "/tmp/other"),
+        ])
+        result = self.run_script(
+            "rename",
+            env_extra={"FAKE_FZF_CHOICE_LINE": "1"},  # picks "old"
+            stdin="new\n",
+        )
+        self.assertEqual(result.returncode, 0, msg=result.stderr)
+        calls = self.tmux_calls()
+        self.assertTrue(
+            any("rename-session -t old new" in c for c in calls),
+            f"expected rename-session -t old new in {calls!r}",
+        )
+        # State should reflect the rename
+        names = self.remaining_sessions()
+        self.assertIn("new", names)
+        self.assertIn("other", names)
+        self.assertNotIn("old", names)
+        self.assertIn("Renamed: old", result.stdout)
+
+
+# -----------------------------------------------------------------------------
+# rename — Boundary / Error cases
+# -----------------------------------------------------------------------------
+
+class TestRenameBoundary(TmuxUtilHarness):
+
+    def test_rename_no_sessions_prints_message(self):
+        self.set_sessions([])
+        result = self.run_script("rename")
+        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()))
+
+    def test_rename_user_cancels_fzf_no_action(self):
+        self.set_sessions([
+            ("old", 0, ["101:zsh"], 950, 1, "/tmp/old"),
+        ])
+        # No FAKE_FZF_CHOICE → fzf exits 130
+        result = self.run_script("rename")
+        self.assertEqual(result.returncode, 0, msg=result.stderr)
+        self.assertFalse(any("rename-session" in c for c in self.tmux_calls()))
+        # State unchanged
+        self.assertEqual(self.remaining_sessions(), ["old"])
+
+    def test_rename_empty_new_name_exits_nonzero(self):
+        self.set_sessions([
+            ("old", 0, ["101:zsh"], 950, 1, "/tmp/old"),
+        ])
+        result = self.run_script(
+            "rename",
+            env_extra={"FAKE_FZF_CHOICE_LINE": "1"},
+            stdin="\n",  # empty new name
+        )
+        self.assertNotEqual(result.returncode, 0)
+        self.assertIn("empty new name", result.stderr)
+        self.assertFalse(any("rename-session" in c for c in self.tmux_calls()))
+
+    def test_rename_same_name_is_noop(self):
+        self.set_sessions([
+            ("old", 0, ["101:zsh"], 950, 1, "/tmp/old"),
+        ])
+        result = self.run_script(
+            "rename",
+            env_extra={"FAKE_FZF_CHOICE_LINE": "1"},
+            stdin="old\n",  # same as current name
+        )
+        self.assertEqual(result.returncode, 0, msg=result.stderr)
+        self.assertIn("same as old", result.stdout)
+        self.assertFalse(any("rename-session" in c for c in self.tmux_calls()))
+
+    def test_rename_conflict_with_existing_exits_nonzero(self):
+        self.set_sessions([
+            ("old", 0, ["101:zsh"], 950, 1, "/tmp/old"),
+            ("taken", 0, ["201:zsh"], 900, 1, "/tmp/taken"),
+        ])
+        result = self.run_script(
+            "rename",
+            env_extra={"FAKE_FZF_CHOICE_LINE": "1"},  # picks "old"
+            stdin="taken\n",  # collides with existing
+        )
+        self.assertNotEqual(result.returncode, 0)
+        self.assertIn("already exists", result.stderr)
+        self.assertFalse(any("rename-session" in c for c in self.tmux_calls()))
+
+
 if __name__ == "__main__":
     unittest.main()