feat(harness): require_phase_or_exit + phase advance/reopen/show (Onda 1.2)

require_phase_or_exit: guard riscritto vs Workflow (load_workflow().phase_name invece
della costante PHASE_NAMES drift). Exit 1 se la sessione e' sotto soglia. Usato da
cte/decision/datamart cmd.

Comandi phase (portati + adattati al modello ThothII, non copia cieca):
- advance: persiste phase_approved; --auto exit 6 se la fase non e' completa
  (contratto col gate)
- reopen: persiste phase_reopened + teardown_to_phase degli artefatti oltre il target
- show: stato sessione (fase corrente, ultime decisioni)

session_dir helper tenuto qui (mirror di session_cmd) per evitare circular import.
_cfg() fa fallback a THT_WORKSPACE env finche' _load_config_or_exit (Onda 1.4) non
sara' portato.

L1: 4 test require_phase_or_exit (allow at/above, exit below, message con nome fase
dal workflow). Suite: 161 passed.

harness/tests/test_require_phase_or_exit.py

@@ -0,0 +1,69 @@
+"""L1: require_phase_or_exit — the phase-gate guard rewritten vs Workflow.
+
+Used by cte_cmd/decision_cmd/datamart_cmd to refuse an action when the session
+hasn't reached a phase yet. The rewrite replaces the old PHASE_NAMES constant
+with load_workflow().phase_name(). Exit code 1 (typer.Exit) when below threshold.
+"""
+import pytest
+import typer
+
+from tht.cli.phase_cmd import require_phase_or_exit
+
+
+def _make_session(tmp_path, current_phase_num: int) -> str:
+    """Build a session dir whose current_phase reads as current_phase_num.
+    current_phase is derived from the ledger fold (cur = last approved + 1), so
+    approving phases 1..N-1 puts the session at phase N."""
+    import json
+
+    s = tmp_path / "sess"
+    s.mkdir()
+    decisions = [
+        {"seq": n, "type": "phase_approved", "subject": f"phase:{n}", "ts": "2025-01-01T00:00:00"}
+        for n in range(1, current_phase_num)
+    ]
+    (s / "review_decisions.jsonl").write_text(
+        "\n".join(json.dumps(d) for d in decisions) + ("\n" if decisions else "")
+    )
+    return "sess"
+
+
+class _StubConfig:
+    """Minimal config: paths.sessions points at tmp_path so session_dir resolves there."""
+
+    def __init__(self, tmp_path):
+        from tht.config import PathsConfig
+
+        self.paths = PathsConfig(sessions=tmp_path)
+
+
+def test_require_phase_or_exit_allows_when_at_or_above(tmp_path):
+    cfg = _StubConfig(tmp_path)
+    session = _make_session(tmp_path, current_phase_num=5)  # at phase 5
+    # require phase 4 -> allowed (cur 5 >= 4), no exit
+    require_phase_or_exit(cfg, session, min_phase=4)
+
+
+def test_require_phase_or_exit_allows_when_exactly_at(tmp_path):
+    cfg = _StubConfig(tmp_path)
+    session = _make_session(tmp_path, current_phase_num=4)  # at phase 4
+    require_phase_or_exit(cfg, session, min_phase=4)  # boundary: allowed
+
+
+def test_require_phase_or_exit_exits_when_below(tmp_path):
+    cfg = _StubConfig(tmp_path)
+    session = _make_session(tmp_path, current_phase_num=3)  # at phase 3
+    with pytest.raises(typer.Exit) as exc_info:
+        require_phase_or_exit(cfg, session, min_phase=5)  # require 5 -> exit 1
+    assert exc_info.value.exit_code == 1
+
+
+def test_require_phase_or_exit_message_uses_workflow_phase_name(tmp_path, capsys):
+    cfg = _StubConfig(tmp_path)
+    session = _make_session(tmp_path, current_phase_num=3)
+    with pytest.raises(typer.Exit):
+        require_phase_or_exit(cfg, session, min_phase=6)  # phase 6 = cte
+    captured = capsys.readouterr()
+    # the message must use the workflow-derived name (not a hardcoded constant)
+    assert "cte" in captured.err.lower()
+    assert "Fase 6" in captured.err