core/agent: parallel tool calls and tool-result char budget

Truncate large tool outputs and run multiple tool calls in one round
concurrently when enabled.

tests/test_agent/test_budget.py

@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+from plyngent.agent.budget import truncate_tool_result
+
+
+def test_truncate_tool_result_short() -> None:
+    assert truncate_tool_result("hello", 100) == "hello"
+
+
+def test_truncate_tool_result_long() -> None:
+    text = "a" * 50
+    out = truncate_tool_result(text, 20)
+    assert out.startswith("a" * 20)
+    assert "truncated" in out
+    assert "30" in out