fix(kg): validate ISO-8601 date formats at MCP boundary

tool_kg_query (as_of), tool_kg_add (valid_from), and tool_kg_invalidate
(ended) accepted any string and forwarded it to SQLite without format
validation. Parameterized queries prevent SQL injection, but invalid
date strings silently produce empty result sets — callers cannot
distinguish "no fact at this time" from "your date format was
unrecognized." This is especially painful for natural-language LLM
callers that synthesize dates like "March 2026" or "Jan 2025".

Add sanitize_iso_date() in config.py alongside the other input
validators. It accepts YYYY, YYYY-MM, and YYYY-MM-DD forms; passes
through None/empty; and raises ValueError with a field-named message
on anything else. Call it from the three kg MCP tool wrappers before
values reach the storage layer so the caller gets a clear error
instead of a silent miss.

Closes #1164

tests/test_config.py

@@ -3,7 +3,13 @@ import json
 import tempfile
 
 import pytest
-from mempalace.config import MempalaceConfig, normalize_wing_name, sanitize_kg_value, sanitize_name
+from mempalace.config import (
+    MempalaceConfig,
+    normalize_wing_name,
+    sanitize_iso_date,
+    sanitize_kg_value,
+    sanitize_name,
+)
 
 
 def test_default_config():
@@ -212,3 +218,65 @@ def test_kg_value_rejects_null_bytes():
 def test_kg_value_rejects_over_length():
     with pytest.raises(ValueError):
         sanitize_kg_value("a" * 129)
+
+
+# --- sanitize_iso_date ---
+
+
+def test_iso_date_accepts_year_only():
+    assert sanitize_iso_date("2026") == "2026"
+
+
+def test_iso_date_accepts_year_month():
+    assert sanitize_iso_date("2026-03") == "2026-03"
+
+
+def test_iso_date_accepts_full_date():
+    assert sanitize_iso_date("2026-03-15") == "2026-03-15"
+
+
+def test_iso_date_passes_through_none():
+    assert sanitize_iso_date(None) is None
+
+
+def test_iso_date_passes_through_empty_string():
+    assert sanitize_iso_date("") == ""
+
+
+def test_iso_date_strips_whitespace():
+    assert sanitize_iso_date("  2026-03-15  ") == "2026-03-15"
+
+
+def test_iso_date_rejects_natural_language():
+    with pytest.raises(ValueError):
+        sanitize_iso_date("March 2026")
+
+
+def test_iso_date_rejects_abbreviated_month():
+    with pytest.raises(ValueError):
+        sanitize_iso_date("Jan 2025")
+
+
+def test_iso_date_rejects_us_format():
+    with pytest.raises(ValueError):
+        sanitize_iso_date("03/15/2026")
+
+
+def test_iso_date_rejects_invalid_month():
+    with pytest.raises(ValueError):
+        sanitize_iso_date("2026-13")
+
+
+def test_iso_date_rejects_invalid_day():
+    with pytest.raises(ValueError):
+        sanitize_iso_date("2026-02-32")
+
+
+def test_iso_date_rejects_non_string():
+    with pytest.raises(ValueError):
+        sanitize_iso_date(20260315)
+
+
+def test_iso_date_error_names_field():
+    with pytest.raises(ValueError, match="valid_from"):
+        sanitize_iso_date("yesterday", "valid_from")