Forking Copilot (Agent Host) chat creates orphaned sessions, broken in VSCode

Type: Bug

This behavior is 100% repro on my machine, whether I use the `/fork` command or click the Fork button on a previous reply.

### Repro steps
1. Start new Copilot (Agent Host) chat session
2. Send a couple test messages
3. **Do either:** Issue `/fork` command, or click Fork button above a previous prompt

### Expected
* New forked chat is created and becomes the active chat
* Chat title is something like `"Forked: "`
* If I click Back on the chat window, or click the chat title, I am able to see my original session and switch to it.

### Actual
* New forked session is created and becomes the active chat
* Title **briefly** updates to `"Forked: ..."`, but then flips back to `"..."` (loses "Forked:" prefix)
* Clicking Back on the chat window should only a **single** session -- no indication there are now 2 distinct sessions.
* Selecting the session title in the overview or via title dropdown appears to open the original chat, not the forked one.
* Forked session now lost

NOTE: If I navigate to the directory in my terminal and run `copilot --resume`, I can see the forked session was created:

...but the fork doesn't show up in VSCode at all:

**This completely breaks the ability to fork when using the Copilot harness in VSCode.**

## System details

VS Code version: Code 1.136.1 (a44adf7f53e00964ab890f9f8758a334f1fc15bc, 2026-09-03T05:06:41Z)
OS version: Windows_NT x64 10.0.26200
Modes:

System Info

|Item|Value|
|---|---|
|CPUs|Intel(R) Core(TM) Ultra 7 165H (22 x 3072)|
|GPU Status|2d_canvas: enabled
GPU0: VENDOR= 0x8086, DEVICE=0x7d55 [Intel(R) Arc(TM) Graphics], DRIVER_VENDOR=Intel, DRIVER_VERSION=32.0.101.6737 *ACTIVE*
GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.8875
Machine model name:
Machine model version:
direct_rendering_display_compositor: disabled_off_ok
gpu_compositing: enabled
multiple_raster_threads: enabled_on
opengl: enabled_on
rasterization: enabled
raw_draw: disabled_off_ok
skia_graphite: disabled_off
trees_in_viz: disabled_off
video_decode: enabled
video_encode: enabled
webgl: enabled
webgpu: enabled
webnn: disabled_off|
|Load (avg)|undefined|
|Memory (System)|63.65GB (7.04GB free)|
|Process Argv|--folder-uri file:///d%3A/NEXT-dcgm --crash-reporter-id 8c2857f5-34d9-47e8-a9b9-099a549006aa|
|Screen Reader|no|
|VM|0%|
Extensions (56)

Name|Identifier|Author|Version
---|---|---|---
rust|1yib.rust-bundle|1YiB|1.0.0
vscode-base64|adamhartford.vscode-base64|adamhartford|0.1.0
AzDO Pull Requests|ankitbko.vscode-pull-request-azdo|ankitbko|1.0.2
gitignore|codezombiech.gitignore|codezombiech|0.10.0
Docker DX|docker.docker|docker|0.18.0
XML Tools|dotjoshjohnson.xml|DotJoshJohnson|2.5.1
Rust Syntax|dustypomerleau.rust-syntax|dustypomerleau|0.6.1
GitHub Codespaces|github.codespaces|GitHub|1.18.16
GitHub Repositories|github.remotehub|GitHub|0.64.0
GitHub Actions|github.vscode-github-actions|github|0.32.3
Go|golang.go|golang|0.56.1
Spreadsheet Viewer|grapecity.gc-excelviewer|GrapeCity|4.2.66
HashiCorp Terraform|hashicorp.terraform|hashicorp|2.40.0
Draw.io Integration|hediet.vscode-drawio|hediet|1.9.0
OpenSCAD Language Support|leathong.openscad-language-support|Leathong|2.0.1
Email|leighlondon.eml|leighlondon|0.4.0
Bash IDE|mads-hartmann.bash-ide-vscode|mads-hartmann|1.43.0
Rainbow CSV|mechatroner.rainbow-csv|mechatroner|3.24.1
Mermaid|mermaidchart.vscode-mermaid-chart|MermaidChart|2.7.6
Git Graph|mhutchie.git-graph|mhutchie|1.30.0
nc-gcode|ml.nc-gcode|ML|0.13.0
Azure Pipelines|ms-azure-devops.azure-pipelines|ms-azure-devops|1.261.1
GitHub Copilot for Azure|ms-azuretools.vscode-azure-github-copilot|ms-azuretools|1.0.231
Azure MCP Server|ms-azuretools.vscode-azure-mcp-server|ms-azuretools|2.0.46
Azure Resources|ms-azuretools.vscode-azureresourcegroups|ms-azuretools|0.12.7
Bicep|ms-azuretools.vscode-bicep|ms-azuretools|0.46.1
Container Tools|ms-azuretools.vscode-containers|ms-azuretools|2.5.0
C# Dev Kit|ms-dotnettools.csdevkit|ms-dotnettools|3.20.199
C#|ms-dotnettools.csharp|ms-dotnettools|2.140.9
.NET Install Tool|ms-dotnettools.vscode-dotnet-runtime|ms-dotnettools|3.1.0
DALEC VSCode Tools|ms-kubernetes-tools.dalec-vscode-tools|ms-kubernetes-tools|0.0.5
Kubernetes|ms-kubernetes-tools.vscode-kubernetes-tools|ms-kubernetes-tools|1.4.1
Python Debugger|ms-python.debugpy|ms-python|2026.6.0
Python|ms-python.python|ms-python|2026.4.0
Pylance|ms-python.vscode-pylance|ms-python|2026.3.1
Python Environments|ms-python.vscode-python-envs|ms-python|1.36.0
Dev Containers|ms-vscode-remote.remote-containers|ms-vscode-remote|0.466.0
Remote - SSH|ms-vscode-remote.remote-ssh|ms-vscode-remote|0.128.0
Remote - SSH: Editing Configuration Files|ms-vscode-remote.remote-ssh-edit|ms-vscode-remote|0.87.0
WSL|ms-vscode-remote.remote-wsl|ms-vscode-remote|0.104.3
Azure Repos|ms-vscode.azure-repos|ms-vscode|0.40.0
Live Preview|ms-vscode.live-server|ms-vscode|0.4.20
PowerShell|ms-vscode.powershell|ms-vscode|2025.4.0
Remote Explorer|ms-vscode.remote-explorer|ms-vscode|0.5.0
Remote Repositories|ms-vscode.remote-repositories|ms-vscode|0.42.0
Remote - Tunnels|ms-vscode.remote-server|ms-vscode|1.5.3
Chat Customizations Evaluations|ms-vscode.vscode-chat-customizations-evaluations|ms-vscode|1.0.8
Live Share|ms-vsliveshare.vsliveshare|ms-vsliveshare|1.1.122
NuGet Gallery|patcx.vscode-nuget-gallery|patcx|1.2.6
YAML|redhat.vscode-yaml|redhat|1.24.0
Git Web Links for VS Code|reduckted.vscode-gitweblinks|reduckted|2.15.1
Terraform doc snippets|run-at-scale.terraform-doc-snippets|run-at-scale|0.6.0
rust-analyzer|rust-lang.rust-analyzer|rust-lang|0.3.3033
Svelte for VS Code|svelte.svelte-vscode|svelte|110.3.1
Even Better TOML|tamasfe.even-better-toml|tamasfe|0.21.2
JWT Debugger|yokawasa.jwt-debugger|yokawasa|0.6.0

A/B Experiments

```
permission_prompt_treatment:1294978
ccr_pr_nudge_control:1319470
vsc_wsm_t:1340632
jidc7660:1370295
auto_mode_control:1367906

```