Adopt existing AGENTS.md

Adoption is a preview-first migration that preserves the source and extracts candidate memories:

memoryledger --json agents adopt AGENTS.md
memoryledger --json agents adopt AGENTS.md \
  --apply --backup --accept \
  --reason "User approved migration."
memoryledger --json agents verify-adoption \
  --source "<exact result.backup path from apply>"
memoryledger --json finalize --accept-all \
  --reason "User approved migration." --export

Preview is read-only. The plan records a source hash, heading inventory, and full-source preservation. Apply prevalidates before changing the ledger, deduplicates retry attempts, and uses deterministic backup names. A root source is expected to shrink to a concise generated file while detailed content moves to linked documents. Verification compares the adopted records and headings to the preserved source; source-changed and backup failures stop the operation.