hardening: mergeOverlay has no defense against malformed remote blocks

packages/opencode/src/memory/prompt.ts:69-80 の mergeOverlay と、packages/opencode/test/skill/release-v0.9.7-adversarial.test.ts の FINDING 接頭辞付きのケースから始めます。これらのテストで現在の動作を再現し、その後、不正なリモート入力を決定的にします。重複したリモート id が両方とも残ってはならず、updated が欠落していても例外をスローしてはなりません。adversarial tests を更新して強化された動作をアサートできれば完了です。

Found while writing v0.9.7 release adversarial tests (packages/opencode/test/skill/release-v0.9.7-adversarial.test.ts).

mergeOverlay (packages/opencode/src/memory/prompt.ts:69-80) has two narrow fragilities, both currently unreachable via the production hydrate/refresh path (which always supplies well-formed RemoteMemoryBlocks via toBlock()), but present in the exported function itself:

1. Duplicate ids within the remote list are not deduplicated against each other. The dedup check only compares new remote entries against local keys (localKeys.has(...)); it never checks a remote entry against remote entries already pushed from the same list. Two remote records sharing one non-sibling scope+id (a corrupted or duplicated overlay fetch) both survive into the merged output.
2. A remote block missing updated throws a TypeError out of the final sort (b.updated.localeCompare(a.updated)) instead of degrading gracefully. toBlock() always guarantees updated via a fallback chain ending in new Date().toISOString(), so this isn't reachable today — but a future caller building RemoteMemoryBlock by hand, or a corrupted overlay cache, would hit it. The memory-read tool's outer try/catch happens to downgrade this to an error response rather than crashing the process, but that's accidental defense-in-depth one layer up, not a property of mergeOverlay itself.

Both are pinned by FINDING:-prefixed tests in the adversarial test file (documenting current behavior, not asserting it's correct). Deferred from v0.9.7 because neither is reachable via the real production path today — worth a hardening pass regardless, since mergeOverlay is exported and its contract implicitly promises a 'deterministic, non-crashing merge' that doesn't fully hold for malformed input.