Failure to write to file

### Describe the bug

Consistent failure to write/edit when telling it to create a plan but don't make any code changes yet.

### Affected version

GitHub Copilot CLI 1.0.39

### Steps to reproduce the behavior

I run copilot cli.
I switch to 'autopilot'
I select my model and I enable all permissions.

I have a prompt ready, this prompt will do some research on a feature I want to implement.
The prompt is a research and planning prompt.
The aim is to create a detailed plan.md file in my project folder.

It does the research it creates the plan, but it cannot save the plan.
It tries to write to various different files, locations (session-state, project location), etc
All writes fail

I see red text how it is failing to make any writes.

This used to work a couple of versions back (1.0.24 etc)

If I run the same prompt again in a new session and I change this text:

"This is only a planning task. Do **not** implement the code yet."

to

"This is a planning task."

Then it works 100% and it creates the plan.md without a single issue.

It is as if the instruction to not make code changes suddenly restricts it from making any file edits.

This use to work fine.

I have a personal and a business github copilot account and I have had the same happen on both accounts on different machines, both are v1.0.39

### Expected behavior

_No response_

### Additional context

_No response_

Start by reproducing the failure in Copilot CLI 1.0.39 with autopilot and the planning-only prompt, then compare it with the prompt that successfully creates plan.md. Trace the file-write path used for the project folder and session-state locations; done means a planning task can create plan.md without allowing code implementation.

索引モデルが issue の本文から書いたものです。