feat(harness): tht sql preview --json + --offset for AGGrid paging (BE-2)

Add inject_limit_offset (tht/execute/limit.py) — pure subquery wrapper that
applies LIMIT/OFFSET non-destructively without clobbering user-supplied LIMITs.
Wire offset param into do_run (pre-processing when offset>0) and add --offset /
--json flags to preview_cmd; JSON mode emits pristine stdout with columns, rows,
execution_ms, truncated, limit, offset. 5 new tests (4 unit + 1 JSON-purity),
219/219 total passing (no regressions).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

harness/tests/test_sql_preview_json.py

@@ -0,0 +1,65 @@
+"""Task 6: `tht sql preview --json` + `--offset` paging.
+
+Pure-logic tests (no DB needed):
+  1. inject_limit_offset — subquery wrapping, non-destructive.
+  2. preview_cmd JSON-purity — monkeypatched do_run, verifies stdout is valid JSON.
+"""
+import json
+
+import pytest
+
+from tht.execute.limit import inject_limit_offset
+
+
+def test_inject_limit_offset_wraps_query():
+    sql = "SELECT a FROM t ORDER BY a"
+    out = inject_limit_offset(sql, limit=10, offset=20)
+    assert "LIMIT 10" in out and "OFFSET 20" in out
+    # original query stays as a subquery (no clobber of an existing user LIMIT)
+    assert "SELECT a FROM t ORDER BY a" in out
+
+
+def test_inject_limit_offset_zero_offset_no_offset_clause():
+    out = inject_limit_offset("SELECT 1", limit=5, offset=0)
+    assert "LIMIT 5" in out
+    assert "OFFSET" not in out
+
+
+def test_inject_limit_offset_strips_trailing_semicolon():
+    out = inject_limit_offset("SELECT 1;", limit=3, offset=0)
+    assert "LIMIT 3" in out
+    # semicolon should not appear in the inner subquery
+    assert ";" not in out
+
+
+def test_inject_limit_offset_default_offset_zero():
+    out = inject_limit_offset("SELECT x FROM y", limit=7)
+    assert "LIMIT 7" in out
+    assert "OFFSET" not in out
+
+
+def test_preview_json_pure_stdout(monkeypatch, tmp_path, capsys):
+    from types import SimpleNamespace
+
+    from tht.cli import sql_cmd
+
+    fake = SimpleNamespace(columns=["a"], rows=[[1], [2]], execution_ms=3, truncated=False)
+    monkeypatch.setattr(sql_cmd, "do_run", lambda *a, **k: fake)
+    monkeypatch.setattr(sql_cmd, "validate_or_exit", lambda *a, **k: SimpleNamespace(ast=None))
+    monkeypatch.setattr(sql_cmd, "require_action", lambda *a, **k: None)
+    monkeypatch.setattr(
+        sql_cmd,
+        "_load_config_or_exit",
+        lambda *a, **k: SimpleNamespace(execution=SimpleNamespace(max_preview_rows=100)),
+    )
+    f = tmp_path / "q.sql"
+    f.write_text("SELECT 1")
+    sql_cmd.preview_cmd(file=f, limit=None, offset=0, session=None, json_out=True, config=None)
+    out = capsys.readouterr().out.strip()
+    data = json.loads(out)  # must parse: stdout pristine
+    assert data["columns"] == ["a"]
+    assert data["rows"] == [[1], [2]]
+    assert data["execution_ms"] == 3
+    assert data["truncated"] is False
+    assert "limit" in data
+    assert data["offset"] == 0