Streamlined upsert with explicit NotFoundError handling and clearer logs

This release focuses on simplifying the upsert flow in src/ops/upsert.ts and making not-found handling and logging more explicit.

Improvements

• Handle not-found explicitly by importing and checking NotFoundError from @fjell/core, proceeding to the creation path when an item is missing (commit 27c7200).
• Simplify control flow by replacing the prior existing-variable and post-fetch branching with a try/catch around get() (commit 27c7200).

Logging

• Add a pre-fetch log message: "Retrieving item by key" before calling get() (commit 27c7200).
• On NotFoundError, log: "Item not found, will create" (commit 27c7200).
• Add targeted error logging for non-not-found errors: "Error getting item during upsert" (commit 27c7200).

Refactoring

• Remove the redundant if(existing) update/else create block and the broad outer catch to tighten error handling (commit 27c7200).