fix(tools): Surface tmux errors from select_window directional navigation

why: session.cmd("next-window" / "previous-window" / "last-window")
discarded the tmux_cmd return value. When tmux emitted an error —
most obviously "last-window" against a session with no previously-
active window, or "next-window" on a single-window session with
wrapping off — stderr was non-empty but the tool silently returned
session.active_window as if the navigation had succeeded. Agents
observed a success response and never noticed the no-op.

what:
- Capture the tmux_cmd from session.cmd(subcommand) and raise
  ToolError if proc.stderr is non-empty, using the subcommand name
  in the message so the agent knows which operation failed.
- Add test_select_window_last_on_single_window_session_raises: a
  freshly-created session has no prior window, so "last-window"
  emits "no last window" on stderr. Verified to FAIL against the
  previous code (silent success) and PASS with the fix.

Scope is deliberately narrow: only the one new call site flagged
by review. Other .cmd() sites in pre-existing tools follow the
same silent pattern and are out of scope for this PR (separate
hygiene follow-up).

Author: tony

src/libtmux_mcp/tools/session_tools.py

@@ -289,7 +289,11 @@ def select_window(
     if subcommand is None:
         msg = f"Invalid direction: {direction!r}. Valid: next, previous, last"
         raise ToolError(msg)
-    session.cmd(subcommand)
+    proc = session.cmd(subcommand)
+    if proc.stderr:
+        stderr = " ".join(proc.stderr).strip()
+        msg = f"tmux {subcommand} failed: {stderr}"
+        raise ToolError(msg)
 
     active_window = session.active_window
     return _serialize_window(active_window)

tests/test_session_tools.py

@@ -244,6 +244,27 @@ def test_select_window_requires_target(mcp_server: Server) -> None:
         select_window(socket_name=mcp_server.socket_name)
 
 
+def test_select_window_last_on_single_window_session_raises(
+    mcp_server: Server, mcp_session: Session
+) -> None:
+    """select_window last with no prior window must surface the tmux error.
+
+    Regression guard: session.cmd("last-window") on a session that has
+    never had a previously-active window emits "no last window" on
+    stderr, but the tool previously discarded the return value and
+    returned the unchanged active window as if the navigation had
+    worked.
+    """
+    # The fixture session is freshly created: there is no previously-
+    # active window for last-window to jump back to.
+    with pytest.raises(ToolError, match="last-window"):
+        select_window(
+            direction="last",
+            session_name=mcp_session.session_name,
+            socket_name=mcp_server.socket_name,
+        )
+
+
 def test_kill_session_requires_target(mcp_server: Server) -> None:
     """kill_session refuses to kill without an explicit target."""
     with pytest.raises(ToolError, match="Refusing to kill"):