fix(cli): fall back to uv pip when pip is unavailable

When nanobot is installed via uv tool install, sys.executable points to
a Python that does not have pip available as a module. _pip_install_argv
and _pip_uninstall_argv always used [sys.executable, -m, pip, ...]
which fails in that environment.

Add _pip_available() helper that checks importlib.util.find_spec('pip').
When pip is not available and uv is on PATH, fall back to:
  uv pip install --python <sys.executable> ...
  uv pip uninstall --python <sys.executable> -y ...
If neither pip nor uv is available, raise CliAppError.

Fixes #4158

tests/cli_apps/test_service.py

@@ -777,3 +777,68 @@ def test_run_blocks_working_dir_outside_workspace(tmp_path: Path) -> None:
 
     with pytest.raises(CliAppError, match="outside the configured workspace"):
         manager.run("gimp", working_dir="/etc", restrict_to_workspace=True)
+
+
+def test_install_uses_uv_pip_when_pip_unavailable(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    manager = _manager(tmp_path)
+    _seed_catalog(manager)
+    calls: list[list[str]] = []
+
+    def fake_run(argv: list[str], *, timeout: int) -> subprocess.CompletedProcess[str]:
+        calls.append(argv)
+        return subprocess.CompletedProcess(argv, 0, stdout="ok", stderr="")
+
+    monkeypatch.setattr(CliAppManager, "_pip_available", staticmethod(lambda: False))
+    monkeypatch.setattr(
+        "nanobot.apps.cli.service.shutil.which",
+        lambda command: "/usr/bin/uv" if command == "uv" else None,
+    )
+    monkeypatch.setattr(manager, "_run_argv", fake_run)
+    monkeypatch.setattr(manager, "_fetch_skill_content", lambda app: None)
+
+    manager.install("gimp")
+
+    assert calls[0][:6] == [
+        "uv",
+        "pip",
+        "install",
+        "--python",
+        sys.executable,
+        "cli-anything-gimp",
+    ]
+
+
+def test_uninstall_uses_uv_pip_when_pip_unavailable(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    manager = _manager(tmp_path)
+    _seed_catalog(manager)
+    manager._save_installed({"suno": {"entry_point": "suno"}})
+    calls: list[list[str]] = []
+
+    def fake_run(argv: list[str], *, timeout: int) -> subprocess.CompletedProcess[str]:
+        calls.append(argv)
+        return subprocess.CompletedProcess(argv, 0, stdout="ok", stderr="")
+
+    monkeypatch.setattr(CliAppManager, "_pip_available", staticmethod(lambda: False))
+    monkeypatch.setattr(
+        "nanobot.apps.cli.service.shutil.which",
+        lambda command: "/usr/bin/uv" if command == "uv" else None,
+    )
+    monkeypatch.setattr(manager, "_run_argv", fake_run)
+
+    manager.uninstall("suno")
+
+    assert calls[0] == [
+        "uv",
+        "pip",
+        "uninstall",
+        "--python",
+        sys.executable,
+        "-y",
+        "suno-cli",
+    ]