Remove existing-leaf B3 fallbacks in AI services

Context

This ticket exists to give a narrow close target to a small, already-verified slice of the broader B3 cleanup tracked by #12461. The broader ticket still has known residuals, so using it as the PR close target would falsely close work that remains open.

Duplicate sweep evidence:

• Knowledge Base semantic search was attempted for the B3 cleanup context but the KB collection was unavailable in this session.
• Local issue/discussion sweep over resources/content/issues and resources/content/discussions found historical swarmHeartbeat.targetSource and AiConfig context but no duplicate for this exact three-file existing-leaf slice.
• Live GitHub REST search returned total_count: 0 for existing-leaf B3 Orchestrator ChromaManager SessionService.

The Problem

ADR 0019 made AiConfig the reactive Provider SSOT. Production AI consumers should read resolved leaves directly. Existing-leaf B3 fallbacks such as ?? null, || 2000, || '<unset>', and provider-name substitutes hide missing or malformed config by silently inventing local defaults at the consumer.

The currently verified removable slice is narrow:

• ai/daemons/orchestrator/Orchestrator.mjs reads AiConfig.orchestrator.swarmHeartbeat.targetSource.
• ai/services/memory-core/managers/ChromaManager.mjs reads aiConfig.summarizationBatchLimit in both count methods.
• ai/services/memory-core/SessionService.mjs reads provider model leaves and aiConfig.summarizationBatchLimit directly for logging/guardrail metadata.

The Architectural Reality

These are production consumers of existing imported config leaves. They do not introduce a new config key, tool contract, or public API surface. The correct substrate is a direct-read cleanup in the consuming services, not a config-template or schema expansion.

Current line anchors on the implementation branch:

• ai/daemons/orchestrator/Orchestrator.mjs:511
• ai/services/memory-core/managers/ChromaManager.mjs:309
• ai/services/memory-core/managers/ChromaManager.mjs:348
• ai/services/memory-core/SessionService.mjs:253
• ai/services/memory-core/SessionService.mjs:577
• ai/services/memory-core/SessionService.mjs:592

The Fix

Remove only the existing-leaf B3 defensive fallbacks in the three files above:

• Replace AiConfig.orchestrator.swarmHeartbeat.targetSource ?? null with the direct leaf.
• Replace aiConfig.summarizationBatchLimit || 2000 with the direct leaf in ChromaManager.
• Replace SessionService provider-model fallbacks and summarizationBatchLimit || 'unset' with direct leaf reads.

Contract Ledger Matrix

Not applicable. This ticket changes no consumed public/config/tool surface and adds no new config leaf. It only removes consumer-side hidden defaults for leaves that already exist in the imported AiConfig authority.

Decision Record Impact

Aligned with ADR 0019, the AiConfig reactive Provider SSOT. This ticket does not amend, supersede, or challenge the ADR.

Acceptance Criteria

• Orchestrator assigns swarmHeartbeatService.targetSource from AiConfig.orchestrator.swarmHeartbeat.targetSource without ?? null or another local default.
• ChromaManager count methods read aiConfig.summarizationBatchLimit directly without || 2000.
• SessionService logging and guardrail metadata read provider model leaves and summarizationBatchLimit directly without local substitutes.
• A focused static sweep over the three touched files returns no existing-leaf B3 fallback patterns for this slice.
• PR evidence explicitly states that broader #12461 residuals remain open and are not closed by this ticket.

Out of Scope

• memorySharing B3 residuals in SummaryService / MemoryService; those are blocked by red PR #12532 and should be cleaned after that lane resolves or folded into it.
• HealthService residuals touched by open self PR #12562.
• PullRequestSyncer pullFilenamePrefix || 'pr-'; the imported config lacks that leaf, so that belongs to a ledgered missing-leaf follow-up rather than a bare-strip cleanup.
• Normalized plain-object fallbacks in KnowledgeBaseIngestionService and generic local config helpers that are not direct AiConfig SSOT reads.

Avoided Traps

• Do not close broad #12461 from this narrow PR; it still contains verified residual work.
• Do not add missing config leaves in the same PR. Existing-leaf cleanup and config-contract expansion are different substrates.
• Do not mutate ignored local overlays to make tests pass; local ai/config.mjs contamination must not become tracked PR scope.

Related

Related: #12461 Related: #12456 Blocked-adjacent: #12532 Conflict-adjacent: #12562

Handoff Retrieval Hints

Retrieval Hint: query_summaries("AiConfig SSOT B3 existing-leaf residuals Orchestrator ChromaManager SessionService #12461") Retrieval Hint: commit range HEAD~1..HEAD on branch codex/12461-b3-existing-leaves.