feat(synthesis): Lead/Provenance model + grounded leads + conjecture generator

- codex/synthesis.py: Lead/Provenance dataclasses, find_connections/gaps/improvements,
  propose_conjectures (status=unverified, quarantined in leads/conjectures/),
  write_leads (grounded→leads/grounded/, conjectures→leads/conjectures/ HARD invariant)
- codex/cli.py: synthesis leads/conjectures/report command group
- codex/config.py: F-13 settings (leads_dir, synthesis_llm_*, synthesis_top_k,
  synthesis_min_grounded_ratio, synthesis_gap_min_coverage)
- tests/synthesis/: 42 tests covering model, grounding, conjecture invariant, quarantine, CLI
- spike/: F-13 spike script + output (live run attempted)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/synthesis/test_model.py

@@ -0,0 +1,87 @@
+"""Tests for the Lead / Provenance datamodel (F-13)."""
+
+from __future__ import annotations
+
+from typing import get_args
+
+import pytest
+
+from codex.synthesis import Lead, LeadKind, LeadStatus, Provenance
+
+
+def test_provenance_requires_bibkey_and_locator() -> None:
+    """Provenance has two required positional fields."""
+    p = Provenance(bibkey="Smith2020", locator="chunk 7")
+    assert p.bibkey == "Smith2020"
+    assert p.locator == "chunk 7"
+
+
+def test_provenance_missing_field_raises() -> None:
+    """Provenance without locator must fail to construct (TypeError)."""
+    with pytest.raises(TypeError):
+        Provenance(bibkey="Smith2020")  # type: ignore[call-arg]
+
+
+def test_lead_required_fields() -> None:
+    """Lead requires id, kind, title, body, provenance, confidence."""
+    lead = Lead(
+        id="L-0001",
+        kind="connection",
+        title="A connection",
+        body="Some body text.",
+        provenance=[Provenance(bibkey="X2020", locator="chunk 1")],
+        confidence=0.8,
+    )
+    assert lead.id == "L-0001"
+    assert lead.kind == "connection"
+    assert lead.status == "unverified"  # default
+    assert lead.suggested_validation == ""  # default
+    assert lead.claims == []  # default factory
+
+
+def test_lead_kind_literals_exact() -> None:
+    """Lead kinds are exactly: connection, gap, improvement, conjecture."""
+    assert set(get_args(LeadKind)) == {"connection", "gap", "improvement", "conjecture"}
+
+
+def test_lead_status_literals_exact() -> None:
+    """Lead statuses are exactly: unverified, spiking, go, no-go."""
+    assert set(get_args(LeadStatus)) == {"unverified", "spiking", "go", "no-go"}
+
+
+def test_lead_provenance_can_be_multiple() -> None:
+    """A Lead may carry multiple provenance pointers."""
+    provs = [
+        Provenance(bibkey="A2020", locator="chunk 1"),
+        Provenance(bibkey="B2021", locator="chunk 7"),
+    ]
+    lead = Lead(
+        id="L-0002",
+        kind="gap",
+        title="t",
+        body="b",
+        provenance=provs,
+        confidence=0.5,
+    )
+    assert len(lead.provenance) == 2
+
+
+def test_lead_missing_provenance_field_raises() -> None:
+    """Constructing Lead without provenance is a TypeError."""
+    with pytest.raises(TypeError):
+        Lead(  # type: ignore[call-arg]
+            id="L-0003",
+            kind="connection",
+            title="x",
+            body="y",
+            confidence=0.5,
+        )
+
+
+def test_lead_id_format_helper() -> None:
+    """The internal _make_lead_id helper produces zero-padded L-XXXX strings."""
+    from codex.synthesis import _make_lead_id
+
+    assert _make_lead_id(1) == "L-0001"
+    assert _make_lead_id(42) == "L-0042"
+    assert _make_lead_id(9999) == "L-9999"