[Security]: session JSON artifacts are written world-readable on unix (0o644 default umask)

Summary: On unix, session JSON artifacts written through JsonFileStore (session transcripts, metadata, token usage) are published with permissions inherited from the process umask — typically 0o644 — making prompt/output content world-readable on multi-user hosts.

Area: Agent runtime / core

Reproduction or evidence: src/crates/services/services-core/src/json_store.rs:306 writes the payload to a temp file (fs::write) and renames it into place without any permission tightening; the file contains no set_permissions/PermissionsExt/0o600 logic anywhere. On a unix host with default umask 022, the published artifact is world-readable (-rw-r--r--).

Environment: BitFun version/commit 32f2427697ee16f253ee6119860a03255c0b3e47 (origin/main); reproducible on any unix host with default umask (verified by code-path analysis; unix CI matrix exercises the fixed behavior).

src/crates/services/services-core/src/json_store.rs:306 から開始し、Unix 上で JsonFileStore がセッションアーティファクトを書き込み、名前を変更する方法を追跡します。0o22 の umask 下で最終的なファイルモードを確認し、セッション JSON ファイルが全ユーザーから読み取り可能ではなく、Unix のチェックに合格した時点で完了を確認します。