--share and --resume flags together overwrite the original session.md instead of appending

### Describe the bug
#### `--share` and `--resume` flags together overwrite the original session.md instead of appending
---

## Run 1
`copilot --share --no-color -p "count to 100"`

> 1
> 2
> 3
> 4
> 5
> 6
> 7
> 8
> 9
> 10
> .... (truncated for sharing issue)
> 100
>
>
> Total usage est: 1 Premium request
> API time spent: 11s
> Total session time: 16s
> Total code changes: +0 -0
> Breakdown by AI model:
> gpt-5.3-codex 17.4k in, 613 out, 8.8k cached (Est. 1 Premium request)
> Session exported to: copilot-session-ac4570b4-6bfd-474e-b69c-1f5befb3e01a.md

---
## Run 2 with --resume
`copilot --share --no-color --resume ac4570b4-6bfd-474e-b69c-1f5befb3e01a -p "count to 200"`

> 1
> 2
> 3
> 4
> 5
> 6
> 7
> 8
> 9
> 10
> .... (truncated for sharing issue)
> 200
>
>
> Total usage est: 1 Premium request
> API time spent: 8s
> Total session time: 1m 27s
> Total code changes: +0 -0
> Breakdown by AI model:
> gpt-5.3-codex 17.7k in, 562 out, 8.8k cached (Est. 1 Premium request)
> Session exported to: copilot-session-ac4570b4-6bfd-474e-b69c-1f5befb3e01a.md

---

## Share session output

> ### 🤖 Copilot CLI Session
>
> > **Session ID:** `ac4570b4-6bfd-474e-b69c-1f5befb3e01a`
> > **Started:** 2/24/2026, 7:53:47 AM
> > **Duration:** 11s
> > **Exported:** 2/24/2026, 7:53:58 AM
>
> _{⏱️ 2s}
>
> ### 👤 User
>
> count to 200
>
> ---
>
> _{⏱️ 11s}
>
> ### 💬 Copilot
>
> 1
> 2
> 3
> 4
> 5
> 6
> 7
> 8
> 9
> 10
> .... (truncated for sharing issue)
> 200
>
>
> ---
>
> _{Generated by [GitHub Copilot CLI](https://github.com/features/copilot/cli)}

### Affected version

GitHub Copilot CLI 0.0.415

### Steps to reproduce the behavior

use --share with --resume flags

### Expected behavior

Append instead of overwrite

### Additional context

_No response_