Prevent "double work", e.g. reading files twice in Duo Workflow

Problem

Duo workflow is currently likely to do double work, e.g. reading the same file during context gathering and then again during execution.

Desired Outcome

Duo workflow only needs to read a file or any other artifact once, not during both context gathering and execution

Implementation Plan

What we tried so far:

1. Keep conversation history on handover (!337 - merged) - Had to be reverted since executor started changing the plan.
2. Improve executor goal prompt and pass conversat... (!374 - closed) - Tell executor specifically to not plan but execute -> this got rid of unintended changes to the plan but did not solve the double work problem

Next step:

Modify planner prompt to take context into account and prevent double work - see #285 (comment 2416683995) for inspiration

Blocked by #284 (closed) as this might change planner prompt as well.