tests: pin ink engine in _make_tui_argv npm-bootstrap tests (post-merge semantic fix)

Main's rewritten test_tui_npm_install.py tests call _make_tui_argv expecting
the Ink/npm flow unconditionally; with the dual-engine dispatch merged in,
_resolve_tui_engine() auto-selects opentui whenever ui-opentui/dist is built
in the repo, routing the call away from the path under test (first subprocess
became 'node --version' instead of 'npm run build'). Pin the engine to ink
via an autouse fixture, mirroring the existing pinning precedent in
test_tui_resume_flow.py.

plugins/plugin_utils.py

@@ -0,0 +1,135 @@
+"""Shared concurrency helpers for plugin authors.
+
+The most common plugin footgun is the lazy process-wide singleton:
+
+    _client = None
+
+    def get_client():
+        global _client
+        if _client is not None:
+            return _client
+        _client = ExpensiveClient(...)   # <-- TOCTOU: two threads both run this
+        return _client
+
+When two threads call ``get_client()`` before the singleton is set, both pass
+the ``is not None`` guard, both run the expensive initialization, and the
+second write clobbers the first — leaking whatever resource the first client
+opened (connections, file handles, background threads).
+
+Multi-threaded agent sessions share one process (delegated tool calls,
+background workers, the self-improvement fork), so this race is reachable in
+practice. Rather than make every plugin author remember to hand-roll
+double-checked locking, this module gives them two thread-safe primitives:
+
+* :func:`lazy_singleton` — decorator for the zero-arg accessor case.
+* :class:`SingletonSlot` — manual slot for accessors that build different
+  instances depending on a config/key argument.
+
+Both are import-light (stdlib ``threading`` only) so any plugin can import
+them without dragging in heavyweight host modules.
+"""
+
+from __future__ import annotations
+
+import functools
+import threading
+from typing import Callable, Generic, Optional, TypeVar
+
+__all__ = ["lazy_singleton", "SingletonSlot"]
+
+T = TypeVar("T")
+
+
+def lazy_singleton(factory: Callable[[], T]) -> Callable[[], T]:
+    """Wrap a zero-argument factory into a thread-safe lazy singleton accessor.
+
+    The wrapped callable returns the same instance on every call; the factory
+    runs exactly once even under concurrent first calls, using double-checked
+    locking. A ``.reset()`` attribute is attached for tests/teardown.
+
+    Example::
+
+        @lazy_singleton
+        def get_client():
+            return ExpensiveClient(load_config())
+
+        client = get_client()   # built once, safe across threads
+        get_client.reset()      # drop the instance (next call rebuilds)
+
+    Note: if the factory raises, no instance is cached and the next call
+    retries (the lock is released either way).
+    """
+    lock = threading.Lock()
+    box: list = []  # one-element [instance]; empty == not yet built
+
+    @functools.wraps(factory)
+    def accessor() -> T:
+        if box:
+            return box[0]
+        with lock:
+            if box:  # re-check inside the lock
+                return box[0]
+            instance = factory()
+            box.append(instance)
+            return instance
+
+    def reset() -> None:
+        with lock:
+            box.clear()
+
+    accessor.reset = reset  # type: ignore[attr-defined]
+    return accessor
+
+
+class SingletonSlot(Generic[T]):
+    """Thread-safe lazy slot for accessors that take a build argument.
+
+    Use this when the cached instance depends on a config/key passed to the
+    accessor (so a bare zero-arg :func:`lazy_singleton` doesn't fit). The slot
+    caches the first successfully-built instance and ignores the argument on
+    subsequent calls — matching the established "first config wins" singleton
+    semantics most plugins already rely on.
+
+    Example::
+
+        _slot: SingletonSlot[Honcho] = SingletonSlot()
+
+        def get_honcho_client(config=None):
+            return _slot.get(lambda: Honcho(**resolve(config)))
+
+        def reset_honcho_client():
+            _slot.reset()
+
+    The factory runs at most once even under concurrent first calls. If the
+    factory raises, nothing is cached and the next call retries.
+    """
+
+    __slots__ = ("_lock", "_value", "_set")
+
+    def __init__(self) -> None:
+        self._lock = threading.Lock()
+        self._value: Optional[T] = None
+        self._set = False
+
+    def get(self, factory: Callable[[], T]) -> T:
+        # Fast path: already built, no lock needed (a set bool + ref read is
+        # atomic under CPython's GIL).
+        if self._set:
+            return self._value  # type: ignore[return-value]
+        with self._lock:
+            if self._set:  # re-check inside the lock
+                return self._value  # type: ignore[return-value]
+            value = factory()
+            self._value = value
+            self._set = True
+            return value
+
+    def peek(self) -> Optional[T]:
+        """Return the cached instance without building it (None if unset)."""
+        return self._value if self._set else None
+
+    def reset(self) -> None:
+        """Drop the cached instance so the next ``get()`` rebuilds it."""
+        with self._lock:
+            self._value = None
+            self._set = False