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.

2 files changed, 124 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
diff --git a/tests/tmux-util/test_tmux_util.py b/tests/tmux-util/test_tmux_util.py
index 283c3c7..24cc335 100644
--- a/tests/tmux-util/test_tmux_util.py
+++ b/tests/tmux-util/test_tmux_util.py
@@ -374,5 +374,108 @@ class TestLsBoundary(TmuxUtilHarness):
         self.assertNotIn(cwd_under_home, result.stdout)
 
 
+# -----------------------------------------------------------------------------
+# go — Normal cases
+# -----------------------------------------------------------------------------
+
+class TestGoNormal(TmuxUtilHarness):
+
+    def test_go_existing_session_outside_tmux_attaches(self):
+        self.set_sessions([
+            ("work", 0, [201], 900, 1, "/tmp/work"),
+        ])
+        # Outside tmux: TMUX env should be unset.
+        env = {k: v for k, v in os.environ.items() if k != "TMUX"}
+        result = subprocess.run(
+            [SCRIPT, "go", "work"],
+            env={**env, "PATH": self.bin_dir + os.pathsep + env.get("PATH", ""),
+                 "FAKE_TMUX_DIR": self.tmp},
+            capture_output=True, text=True, timeout=10,
+        )
+        self.assertEqual(result.returncode, 0, msg=result.stderr)
+        calls = self.tmux_calls()
+        # has-session should be the first existence check (exact-match form)
+        self.assertTrue(any("has-session" in c and "work" in c for c in calls),
+                        f"expected has-session probe in {calls!r}")
+        self.assertTrue(any("attach-session -t work" in c for c in calls),
+                        f"expected attach-session -t work in {calls!r}")
+        # Must NOT create a new session
+        self.assertFalse(any("new-session" in c for c in calls),
+                         f"unexpected new-session in {calls!r}")
+
+    def test_go_existing_session_inside_tmux_switches_client(self):
+        self.set_sessions([
+            ("work", 0, [201], 900, 1, "/tmp/work"),
+        ])
+        result = self.run_script("go", "work", env_extra={"TMUX": "/tmp/fake-tmux-socket,1234,0"})
+        self.assertEqual(result.returncode, 0, msg=result.stderr)
+        calls = self.tmux_calls()
+        self.assertTrue(any("switch-client -t work" in c for c in calls),
+                        f"expected switch-client in {calls!r}")
+        # attach-session shouldn't be called inside tmux
+        self.assertFalse(any("attach-session" in c for c in calls),
+                         f"unexpected attach-session in {calls!r}")
+
+    def test_go_missing_session_outside_tmux_creates(self):
+        self.set_sessions([])
+        env = {k: v for k, v in os.environ.items() if k != "TMUX"}
+        result = subprocess.run(
+            [SCRIPT, "go", "fresh"],
+            env={**env, "PATH": self.bin_dir + os.pathsep + env.get("PATH", ""),
+                 "FAKE_TMUX_DIR": self.tmp},
+            cwd="/tmp",
+            capture_output=True, text=True, timeout=10,
+        )
+        self.assertEqual(result.returncode, 0, msg=result.stderr)
+        calls = self.tmux_calls()
+        # new-session with -s and -c pointing at the subprocess's cwd
+        self.assertTrue(
+            any("new-session" in c and "-s fresh" in c and "-c /tmp" in c
+                for c in calls),
+            f"expected new-session -s fresh -c /tmp in {calls!r}",
+        )
+
+    def test_go_missing_session_inside_tmux_creates_detached_then_switches(self):
+        self.set_sessions([])
+        env = os.environ.copy()
+        env.update({
+            "PATH": self.bin_dir + os.pathsep + env.get("PATH", ""),
+            "FAKE_TMUX_DIR": self.tmp,
+            "TMUX": "/tmp/fake-tmux-socket,1234,0",
+        })
+        result = subprocess.run(
+            [SCRIPT, "go", "fresh"],
+            env=env,
+            cwd="/tmp",
+            capture_output=True, text=True, timeout=10,
+        )
+        self.assertEqual(result.returncode, 0, msg=result.stderr)
+        calls = self.tmux_calls()
+        self.assertTrue(
+            any("new-session" in c and "-d" in c and "-s fresh" in c and "-c /tmp" in c
+                for c in calls),
+            f"expected detached new-session in {calls!r}",
+        )
+        self.assertTrue(any("switch-client -t fresh" in c for c in calls),
+                        f"expected switch-client after create in {calls!r}")
+
+
+# -----------------------------------------------------------------------------
+# go — Error / Boundary cases
+# -----------------------------------------------------------------------------
+
+class TestGoErrors(TmuxUtilHarness):
+
+    def test_go_with_no_name_exits_nonzero(self):
+        result = self.run_script("go")
+        self.assertNotEqual(result.returncode, 0)
+        self.assertIn("missing session name", result.stderr)
+
+    def test_go_with_empty_string_name_exits_nonzero(self):
+        result = self.run_script("go", "")
+        self.assertNotEqual(result.returncode, 0)
+        self.assertIn("missing session name", result.stderr)
+
+
 if __name__ == "__main__":
     unittest.main()