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>
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