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>

spike/spike_output.json

@@ -0,0 +1,10 @@
+{
+  "live": true,
+  "leak": false,
+  "counts": {
+    "connection": 0,
+    "gap": 0,
+    "conjecture": 0
+  },
+  "leads_dir": "/Users/tarikmoussa/Desktop/codex-py/spike/spike_output"
+}