docs: implementation plan for Phase 1b cycle 2

Mandatory age encryption + gzip transport on snapshot.write/read,
new set-encryption-passphrase command, and a missing-passphrase
precheck on poll. Branch + draft-PR wrap-up follows the cycle-1
pattern.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: tanem