core/prompting+tools: shared ask/choose/form and chat tools

Add process-wide prompting backends for interactive human I/O; wire CLI
limit/confirm/workspace prompts through it. Expose ask_user, choose_user,
and form_user under tools/chat and include them in DEFAULT_TOOLS.

tests/test_tools/test_chat_tools.py

@@ -0,0 +1,66 @@
+from __future__ import annotations
+
+import json
+
+from plyngent.agent import ToolRegistry
+from plyngent.prompting import NonInteractiveBackend, temporary_backend
+from plyngent.tools.chat import CHAT_TOOLS, ask_user, choose_user, form_user
+from tests.test_prompting import ScriptedBackend
+
+
+async def test_ask_user_tool() -> None:
+    backend = ScriptedBackend(["42"])
+    with temporary_backend(backend):
+        registry = ToolRegistry([ask_user])
+        out = await registry.execute("ask_user", '{"question": "Answer?"}')
+    assert out == "42"
+
+
+async def test_choose_user_tool_index() -> None:
+    backend = ScriptedBackend(["1"])
+    with temporary_backend(backend):
+        registry = ToolRegistry([choose_user])
+        out = await registry.execute(
+            "choose_user",
+            json.dumps(
+                {
+                    "question": "Pick",
+                    "options": json.dumps(["alpha", "beta"]),
+                    "allow_custom": False,
+                }
+            ),
+        )
+    assert out == "alpha"
+
+
+async def test_choose_user_bad_options() -> None:
+    registry = ToolRegistry([choose_user])
+    out = await registry.execute(
+        "choose_user",
+        json.dumps({"question": "Pick", "options": "not-json"}),
+    )
+    assert out.startswith("error:")
+
+
+async def test_form_user_tool() -> None:
+    backend = ScriptedBackend(["ncbm"], confirms=[True])
+    fields = json.dumps([{"name": "user", "prompt": "User?"}])
+    with temporary_backend(backend):
+        registry = ToolRegistry([form_user])
+        out = await registry.execute(
+            "form_user",
+            json.dumps({"title": "Setup", "fields": fields, "confirm_submit": True}),
+        )
+    assert json.loads(out) == {"user": "ncbm"}
+
+
+async def test_chat_tools_in_default_list() -> None:
+    names = {t.name for t in CHAT_TOOLS}
+    assert names == {"ask_user", "choose_user", "form_user"}
+
+
+async def test_ask_user_non_interactive_error() -> None:
+    with temporary_backend(NonInteractiveBackend()):
+        registry = ToolRegistry([ask_user])
+        out = await registry.execute("ask_user", '{"question": "hi"}')
+    assert out.startswith("error:")