Document the beta17 release closeout and vacuum precedent

[pathscale]

Summary

• record that WorkTable PR 87, wt-benchmarks PR 6, crates.io publication, support.cafe runtime validation, and AgencyZero PR 205 are complete
• replace the stale beta17 blocker language with the remaining post-release administration decisions
• document comparable maintenance policies in InnoDB, PostgreSQL, and RocksDB, and distinguish them from the per-table mutation lease plus epoch check/recheck gate

Validation

• ./scripts/ci-local.sh
• all default, versioned-publication, all-feature, test, benchmark-smoke, formatting, and clippy jobs passed against local master sources