[P3] Audit log rotation (currently unbounded)

[Railly]

Context

`~/.sunat/audit/YYYY-MM.jsonl` grows unbounded. After a year of daily emissions, the file gets large and `cpe doctor stale_pendings` scan slows down.

Scope

• Per-month JSONL files (already kind of by date) but explicit rotation
• `sunat audit compact` — move >12 months old to compressed archive
• `sunat audit prune --before YYYY-MM` — delete archived months
• Default policy: compact at 6 months, archive at 24 months, never auto-delete

Success criteria

• `stale_pendings` doctor check stays fast even after years of emissions
• Operator can recover archived months on demand

Why P3

Won't matter for ~2 years of normal use. File system is forgiving.

[Railly]

Draft PR opened: #26