fix(security): eliminate SQL string formatting in execute() calls

Closes #1911

- insights.py: Pre-compute SELECT queries as class constants instead of
  f-string interpolation at runtime. _SESSION_COLS is now evaluated once
  at class definition time.
- hermes_state.py: Add identifier quoting and whitelist validation for
  ALTER TABLE column names in schema migrations.
- Add 4 tests verifying no injection vectors in SQL query construction.

tests/test_sql_injection.py

@@ -0,0 +1,43 @@
+"""Tests that verify SQL injection mitigations in insights and state modules."""
+
+import re
+
+from agent.insights import InsightsEngine
+
+
+def test_session_cols_no_injection_chars():
+    """_SESSION_COLS must not contain SQL injection vectors."""
+    cols = InsightsEngine._SESSION_COLS
+    assert ";" not in cols
+    assert "--" not in cols
+    assert "'" not in cols
+    assert "DROP" not in cols.upper()
+
+
+def test_get_sessions_all_query_is_parameterized():
+    """_GET_SESSIONS_ALL must use a ? placeholder for the cutoff value."""
+    query = InsightsEngine._GET_SESSIONS_ALL
+    assert "?" in query
+    assert "started_at >= ?" in query
+    # Must not embed any runtime-variable content via brace interpolation
+    assert "{" not in query
+
+
+def test_get_sessions_with_source_query_is_parameterized():
+    """_GET_SESSIONS_WITH_SOURCE must use ? placeholders for both parameters."""
+    query = InsightsEngine._GET_SESSIONS_WITH_SOURCE
+    assert query.count("?") == 2
+    assert "started_at >= ?" in query
+    assert "source = ?" in query
+    assert "{" not in query
+
+
+def test_session_col_names_are_safe_identifiers():
+    """Every column name listed in _SESSION_COLS must be a simple identifier."""
+    cols = InsightsEngine._SESSION_COLS
+    identifiers = [c.strip() for c in cols.split(",")]
+    safe_identifier = re.compile(r"^[a-zA-Z_][a-zA-Z0-9_]*$")
+    for col in identifiers:
+        assert safe_identifier.match(col), (
+            f"Column name {col!r} is not a safe SQL identifier"
+        )