fix: resolve rust CI clippy failures in memory API sorting

[Copilot]

PR #158 failed the rust check due to clippy::unnecessary_sort_by in src-tauri/src/memory/api.rs. This updates the flagged sort sites to clippy-compliant key-based sorting without changing ordering semantics.

• CI failure scope

  • Addressed six clippy errors in memory query/context assembly paths (list_sessions, query_recent_errors, and get_context collections).

• Code changes

  • Replaced comparator closures (sort_by) with sort_by_key.
  • Used std::cmp::Reverse(...) for descending sorts to preserve existing behavior.

• Behavioral intent

  • No functional change to returned ordering; this is a mechanical refactor to satisfy strict -D warnings clippy policy.

// before
sessions.sort_by(|a, b| b.started_at.cmp(&a.started_at));

// after
sessions.sort_by_key(|session| std::cmp::Reverse(session.started_at));