fix: intermittent Postgres deadlock for concurrent memory searches

[supreme-gg-gg]

Close #2135

Fixes intermittent Postgres deadlocks when concurrent memory searches update overlapping rows' access_count (e.g. PrefetchMemoryTool fanning out per-sentence searches). Row locks are now acquired in id order, and access-count increments are best-effort so a bookkeeping failure no longer fails the search.

The root cause is described in #2135 but in short when two concurrent search requests return results like ["memory_1", "memory_2"] and ["memory_2", "memory_1"], it will result in a deadlock when trying to update the rows.

I've added a concurrency test that reproduced the error without this fix to the query and passed after the fix.

[Copilot]

Pull request overview

This PR addresses intermittent PostgreSQL deadlocks triggered by concurrent memory searches that increment memory.access_count on overlapping row sets. It makes row-lock acquisition deterministic (by id order) and changes access-count increments to be best-effort so memory search results are not blocked by bookkeeping failures.

Changes:

• Update IncrementMemoryAccessCount to lock rows in a deterministic id order before updating to prevent deadlocks between concurrent overlapping increments.
• Make SearchAgentMemory ignore (and log) access-count increment failures so searches still return results.
• Add a concurrency-focused database test to catch regressions around concurrent search/access-count updates.

Reviewed changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
go/core/internal/database/queries/memory.sql	Changes access-count increment SQL to lock rows in id order before updating.
go/core/internal/database/gen/querier.go	Regenerates sqlc interface to reflect the updated query/comments.
go/core/internal/database/gen/memory.sql.go	Regenerates sqlc output with the updated SQL for access-count increments.
go/core/internal/database/client_postgres.go	Makes access-count increments best-effort during memory search (no longer fail the search).
go/core/internal/database/client_test.go	Adds a concurrent search test intended to reproduce/prevent the deadlock scenario.

Files not reviewed (2)

• go/core/internal/database/gen/memory.sql.go: Generated file
• go/core/internal/database/gen/querier.go: Generated file

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