Add local maintenance pruning

[yvette-carlisle]

Summary

• add decodex maintenance prune with dry-run/apply reports for logs, agent evidence, backups, runtime protocol-event compaction, and WAL checkpointing
• run the auto-safe subset from decodex serve at startup and periodically while preserving runtime protocol events
• persist compacted protocol-event summaries and document the retention boundary

Verification

• cargo test -p decodex maintenance --lib
• cargo run -p decodex --bin decodex -- maintenance prune --help
• cargo run -p decodex --bin decodex -- maintenance prune --dry-run --json
• python3 /Users/x/.codex/plugins/cache/hack-ink/playbook/0.3.0/scripts/semantic_drift_audit.py --repo .
• cargo make check