docs(memory): document src/services/memory/index.js and fix save race condition

- Added JSDoc and inline comments to `src/services/memory/index.js` to clarify module flow and internal invariants.
- Fixed a race condition in `performSave` where `currentSavePromise` was cleared before the pending save was chained, potentially allowing concurrent writes.
- Created `docs/memory-service-index-overview.md` with high-level architecture.
- Verified changes with `npm test src/services/memory/index.js` and `npm run lint`.
- Updated `TEST_LOG` and `task-logs` to reflect the fix.

Co-authored-by: PR0M3TH3AN <[email protected]>

src/decisions/DECISIONS_2026-02-27T03-12-00Z.md

@@ -0,0 +1,23 @@
+---
+agent: docs-code-investigator
+cadence: daily
+run-start: 2026-02-27T03:12:00Z
+---
+# Decisions - docs-code-investigator - 2026-02-27T03-12-00Z
+
+Prompt: src/prompts/daily/docs-code-investigator.md
+
+## Decision: Document `src/services/memory/index.js`
+
+### Context
+`src/services/memory/index.js` is a large file (565 LOC) that serves as the main entry point for the memory service, orchestrating multiple sub-modules (`ingestor`, `retriever`, `pruner`). It has complex internal state (memory store caching, save serialization).
+
+### Rationale
+-   High complexity: It manages async persistence with a manual lock/queue mechanism (`performSave`, `saveMemoryStore`).
+-   Critical path: All memory operations pass through here.
+-   Maintenance risk: Future refactors could easily break the save serialization or cache invalidation logic if not clearly documented.
+
+### Implementation Details
+-   Will add comprehensive JSDoc for all public exports.
+-   Will add a detailed module-level comment explaining the flow and invariants.
+-   Will document the `performSave` / `saveMemoryStore` pattern specifically to prevent race conditions in future edits.