fix(memory): make auto-prune observable and age-safe

[Grivn]

Summary

• add a configurable auto-prune grace period with a safe 24-hour default
• base grace on physical local insertion time (stored_at), not historical created_at
• return exact auto_pruned_ids from remember/import operations
• make prune audit records, edge cleanup, soft deletion, and the triggering write atomic
• include the trigger insight, active count, limit, and minimum age in audit detail
• document the behavior and configuration in English and Chinese

MNEMON_AUTO_PRUNE_MIN_AGE accepts a Go duration, an integer number of days, or 0 to disable the grace period. Invalid and negative values fall back safely to the default.

The nullable stored_at migration includes a backfill, an index, and a trigger for legacy/external writers. This protects newly imported historical records: an old event timestamp no longer makes a row immediately eligible for pruning after it first enters the local store.

Relationship to existing work

This is a broader independent solution than #108. Both address the newborn-insight grace period, but this PR additionally handles historical imports through stored_at, exposes exact pruned IDs, and makes audit/write behavior transactional. It preserves the existing prune operation name introduced by #96 and the configurable insight ceiling already on master from #109.

If maintainers prefer #108's opt-in/default-off policy, the default and environment naming can be adjusted without changing the transaction or stored_at design.

Verification

• focused migration, legacy-writer, historical-import, grace-period, and audit-rollback tests
• make test
• 152/152 CLI E2E assertions
• go test -race ./internal/memory/store ./cmd/memory
• the external mnemon-sync convergence harness passes against the migrated schema
• git diff --check

Closes #83.