Drop orphaned Memory Core backupRetention leaf

Context

PR #12542 completes #12541 by moving the maintenance backup/defrag retention resolvers to direct ADR-0019-aligned reads from the top-level maintenance subtree:

aiConfig.maintenance.backup.retention
aiConfig.maintenance.defrag.snapshotRetention

That resolves the B3 optional-chain / fallback cleanup, but it leaves a non-blocking follow-up: after #12542 merges, the Memory Core backupRetention config leaf is no longer consumed by backup.mjs.

V-B-A before filing:

• PR #12542 is APPROVED, CLEAN, and all checks green at f0223ce60.
• On the #12542 branch, rg -n "backupRetention|memoryCoreConfig\.backupRetention|resolveBackupRetention" ai/scripts/maintenance/backup.mjs ai/mcp/server/memory-core/config.mjs ai/mcp/server/memory-core/config.template.mjs test/playwright/unit/ai/scripts/maintenance/backup-retention.spec.mjs learn shows the runtime resolver now reads resolveBackupRetention() from top-level maintenance config; remaining backupRetention references are Memory Core config/template declarations and stale test commentary.
• gh issue list --state all --search "backupRetention orphan Memory Core config leaf" returned no equivalent issue.
• gh pr list --state open --search "backupRetention Memory Core maintenance AiConfig" returned only #12542, not a cleanup PR.
• ask_knowledge_base(type='ticket', query='backupRetention Memory Core maintenance orphan AiConfig ticket') surfaced historical #11663 context, not an active duplicate.

Problem

Once #12542 lands, ai/mcp/server/memory-core/config.mjs and config.template.mjs still advertise backupRetention, but the only live backup retention reader has moved to aiConfig.maintenance.backup.retention.

Keeping the Memory Core leaf after its reader is removed creates a misleading operator config surface: an operator can set memoryCore.backupRetention and reasonably expect it to affect bundle retention, but the maintenance resolver will no longer consult it.

Architectural Reality

The maintenance backup policy now belongs to the top-level AI maintenance subtree. Memory Core still owns storage paths, Chroma coordinates, graph paths, and service-specific config. It should not retain a dead backup policy leaf after bundle retention is governed by the maintenance subtree.

This ticket is dependent on #12542. Before #12542 merges, dev still contains the old fallback reader, so implementation must wait for the approved PR to land or else branch from a post-merge dev.

Fix

After #12542 is merged to dev:

1. Remove the backupRetention leaf from:
   • ai/mcp/server/memory-core/config.mjs
   • ai/mcp/server/memory-core/config.template.mjs
2. Update touched test/commentary surfaces so they no longer describe deployments without backupRetention as a live Memory Core config mode.
3. Verify no tracked runtime/source docs still imply memoryCoreConfig.backupRetention controls bundle retention.

Do not add a new fallback, alias, or migration shim. The top-level maintenance subtree is the source of truth.

Contract Ledger

Decision Record Impact

Aligned with ADR 0019: consumed config must have a single source of truth, and missing config should fail loudly at the direct resolved-leaf read site. This ticket removes a dead secondary config source rather than preserving a dual-source fallback.

Acceptance Criteria

• #12542 has merged before implementation begins, or the implementation branch is explicitly based on post-#12542 dev.
• backupRetention is removed from ai/mcp/server/memory-core/config.mjs and ai/mcp/server/memory-core/config.template.mjs.
• No tracked runtime source still reads or documents memoryCoreConfig.backupRetention as a bundle retention control. Historical resources/content/** archives may remain unchanged.
• Existing maintenance backup retention behavior remains governed by aiConfig.maintenance.backup.retention.
• Focused maintenance retention tests still pass.
• npm run ai:lint-config-template-ssot still passes.

Out of Scope

• Changing retention defaults.
• Changing backup bundle semantics or atomic-bundle behavior.
• Reworking historical issue/PR archive content under resources/content/**.
• Implementing before #12542 lands.

Related

• Parent: #12461
• Prerequisite PR: #12542
• Source ticket: #12541
• Historical origin: #11663
• ADR: learn/agentos/decisions/0019-aiconfig-reactive-provider-ssot.md

Origin Session ID

dcdaac0b-9ae0-45b5-b4da-da39541af497

Retrieval Hint

backupRetention Memory Core maintenance orphan AiConfig #12542 f0223ce60