Resolves #15126

SessionService.findSessionsToSummarize() now folds each Chroma page directly into bounded per-session maps instead of retaining every memory and summary metadata row until the all-time scans finish. Pagination, timestamp normalization, eligibility gates, candidate ordering, and last-summary-row semantics remain in place.

Evidence: L2 — a deterministic page-expiry fixture consumes 8,005 memory rows across 5 pages and 2,001 summary rows across 2 pages while retaining only 5 distinct session aggregates plus the active page; current-session, externally-active, reconciled, mismatch, missing-summary, newest-first, churn, future-skew, and unparseable-timestamp behavior is covered. L2 required — this is an internal service allocation-shape change with no user-facing runtime surface. No residuals.

Deltas from ticket

None substantive. The regression fixture additionally pins the existing current-session and externally-active exclusions so parity does not depend on the older live A2A fixture.

Test Evidence

• npm run agent-preflight -- --no-fix ai/services/memory-core/SessionService.mjs test/playwright/unit/ai/services/memory-core/SessionService.SummarizePagination.spec.mjs — passed.
• node --check on both changed modules — passed.
• npm run test-unit -- test/playwright/unit/ai/services/memory-core/SessionService.SummarizePagination.spec.mjs test/playwright/unit/ai/services/memory-core/SessionService.ChurnCooldown.spec.mjs --workers=1 — 8/8 passed against an ephemeral Chroma instance.
• Full npm run test-unit — 7,077 passed, 5 skipped, 27 failed; all 3 tests in the touched pagination spec passed. The remaining failures span live-fixture/environment surfaces. Baseline was not established, so they are disclosed rather than labeled pre-existing.
• Isolated legacy QueryReRanker.spec.mjs --grep findSessionsToSummarize — 3/5 passed. One failing external-activity assertion stops before the touched method; the stale-session fixture no longer returns the caller-supplied session id from its tool write. The new hermetic fixture pins both eligibility boundaries directly.
• Surface: SessionService.findSessionsToSummarize() — memory and summary discovery now aggregate page-locally; returned candidate semantics stay covered by deterministic unit tests.
• Non-CI coverage: None found — this internal Memory Core service path has focused unit coverage and no whitebox E2E surface.

Post-Merge Validation

• Confirm the focused SessionService unit cohort remains green on merged dev.
• Observe the next scheduled drift sweep for unchanged candidate behavior and absence of discovery errors.

Authored by Emmy (GPT-5.6 Sol, Codex Desktop). Session f95e01ff-ba36-409a-98af-573263fab247.