Remove maintenance B3 AiConfig defensive reads

Context

Sub-leaf of #12461 (B3 defensive reads of the AiConfig SSOT). The broad parent identifies scripts/maintenance as one cleanup cluster, but the current open B3 PRs cover GitLab (#12528) and ConceptDiscoveryService (#12539), not this two-file maintenance-script surface.

Duplicate / collision sweep performed 2026-06-04:

• GitHub issue searches for maintenance B3 AiConfig backup defrag and backup.mjs defragChromaDB AiConfig returned no equivalent open child ticket.
• Open PR search for #12461 shows #12528 and #12539 only.
• Local resources/content/{issues,discussions} sweep found historical maintenance-script context, but no active equivalent B3 cleanup ticket for this cluster.
• origin/dev grep shows the relevant live B3 reads at ai/scripts/maintenance/backup.mjs:140 and ai/scripts/maintenance/defragChromaDB.mjs:152.

The Problem

ADR 0019 classifies defensive ?. reads on the AiConfig SSOT as B3: the config provider guarantees the resolved tree, so consumers should read resolved leaves at the use site and fail loud if the subtree is missing.

The maintenance scripts still defend these reads:

• resolveBackupRetention() uses aiConfig.maintenance?.backup?.retention.
• resolveDefragSnapshotRetention() uses aiConfig.maintenance?.defrag?.snapshotRetention.

Those functions are operator-facing maintenance-script helpers. Keeping B3 guards here weakens the fail-loud contract precisely on the scripts operators run for backup and Chroma defrag safety.

The Architectural Reality

These scripts are legitimate AI entrypoints that already import Tier-1 AiConfig from ai/config.mjs. Per ADR 0019 §5, entrypoints read the resolved leaves directly. No new config surface or fallback policy is needed; the leaf defaults live in the AiConfig provider.

This is a narrow read-site cleanup, not a retention-policy redesign. The existing fallback behavior inside the returned retention objects remains owned by the leaf defaults and function-local destructuring/default handling.

The Fix

Change only the maintenance B3 read sites:

• ai/scripts/maintenance/backup.mjs: aiConfig.maintenance?.backup?.retention -> aiConfig.maintenance.backup.retention.
• ai/scripts/maintenance/defragChromaDB.mjs: aiConfig.maintenance?.defrag?.snapshotRetention -> aiConfig.maintenance.defrag.snapshotRetention.

Run targeted verification for the affected scripts and the B3 grep for these files.

Contract Ledger

Decision Record impact

aligned-with ADR 0019.

Acceptance Criteria

• ai/scripts/maintenance/backup.mjs has no B3 optional chaining on the AiConfig maintenance subtree.
• ai/scripts/maintenance/defragChromaDB.mjs has no B3 optional chaining on the AiConfig maintenance subtree.
• No hidden default or fallback is introduced to replace the removed defensive reads.
• Targeted tests or existing focused script coverage pass; if no focused coverage exists, the PR body states that explicitly and provides the grep evidence.

Out of Scope

• Changing backup or defrag retention semantics.
• Changing AiConfig leaf defaults.
• Cleaning other #12461 B3 clusters.
• B3/B4 test-spec cleanup, which remains coupled to #12435.

Related

Parent: #12461. Epic: #12456. Authority: ADR 0019.

Origin Session ID: dcdaac0b-9ae0-45b5-b4da-da39541af497

Retrieval Hint: maintenance B3 AiConfig backup defragChromaDB ADR 0019