feat: add loop guard to halt repeated-identical tool call loops

Introduces `pallas.loop_guard` module that detects and halts agentic loops
where the same `(tool, args) → result` repeats consecutively, preventing
wasted LLM turns when upstream MCP servers return contradictory data.

- Add per-request `ToolRunnerHooks` tracking rolling tool-call signatures
- Halt loop after `loop_repeat_threshold` consecutive repeats (default 3)
- Collapse `max_iterations` on halt to terminate without further LLM call
- Append user-facing explanation to the turn with `stop_reason=endTurn`
- Expose `pallas_agent_loop_aborted_total{agent,reason}` counter
- Add per-agent `max_iterations` and `loop_repeat_threshold` config
- Document guard behavior, metric, and alerting query

tests/test_log.py

@@ -0,0 +1,55 @@
+"""Tests for ``pallas.log._JSONFormatter``.
+
+The formatter must serialize caller-supplied ``extra={...}`` fields (the
+loop guard and other diagnostics rely on this) while never emitting the
+internal ``LogRecord`` bookkeeping attributes.
+"""
+from __future__ import annotations
+
+import json
+import logging
+
+from pallas.log import _JSONFormatter
+
+
+def _format(msg: str, **extra) -> dict:
+    record = logging.LogRecord(
+        name="pallas.test",
+        level=logging.WARNING,
+        pathname=__file__,
+        lineno=1,
+        msg=msg,
+        args=(),
+        exc_info=None,
+    )
+    for key, value in extra.items():
+        setattr(record, key, value)
+    return json.loads(_JSONFormatter().format(record))
+
+
+def test_extra_fields_are_serialized():
+    out = _format(
+        "agentic loop halted",
+        event="loop_halt",
+        tool="kairos-update_task",
+        repeat_count=3,
+        result_preview="COMPLETED but 0%",
+    )
+    assert out["message"] == "agentic loop halted"
+    assert out["event"] == "loop_halt"
+    assert out["tool"] == "kairos-update_task"
+    assert out["repeat_count"] == 3
+    assert out["result_preview"] == "COMPLETED but 0%"
+
+
+def test_standard_attributes_are_not_leaked():
+    out = _format("plain message")
+    for noise in ("msg", "args", "levelno", "pathname", "lineno", "funcName"):
+        assert noise not in out
+    assert out["level"] == "WARNING"
+    assert out["logger"] == "pallas.test"
+
+
+def test_non_serializable_extra_does_not_crash():
+    out = _format("with object", obj=object())
+    assert "obj" in out  # coerced via default=str, not dropped or raised