Copy to clipboard silently fails on Windows

### Describe the bug

## Describe the bug

Copying agent output to the clipboard stopped working on Windows. The copy operation
appears to succeed (no error shown) but paste yields the previous clipboard contents —
the clipboard is not updated. This works correctly in 1.0.48.

---

## Affected version

```
GitHub Copilot CLI 1.0.57-5.
Run 'copilot update' to check for updates.
```

Broken: **1.0.55 through 1.0.57** (current stable as of 2026-06-01)
Working: **1.0.48** and below
Not tested: **1.0.49 through 1.0.54** (may work; the first relevant change — cell-based
renderer — appears in 1.0.55)

---

### Affected version

GitHub Copilot CLI 1.0.57-5

### Steps to reproduce the behavior

## Steps to reproduce the behavior

1. Install Copilot CLI 1.0.55 or later on Windows
2. Start a session and generate any code or text output
3. Attempt to copy the output to the clipboard using the CLI's native copy function
4. Attempt to paste elsewhere — clipboard retains its prior content; the copy had no effect

---

### Expected behavior

## Expected behavior

The copy function updates the Windows clipboard with the selected agent output, replacing
any prior clipboard content and making it available to paste into other applications.

---

### Additional context

## Additional context

**Root cause candidate — 1.0.55-6 changelog:**

> "Cell-based terminal renderer is now enabled for all users by default"

This is the most significant change to the terminal I/O subsystem between 1.0.48 and
1.0.56. The cell-based renderer is a fundamental replacement of the rendering pipeline,
and clipboard operations (OSC 52 escape sequences or system clipboard integration) are
tightly coupled to the rendering layer.

**Supporting evidence from the same 1.0.55 release cycle:**
- 1.0.55-1: "Clipboard correctly copies CJK and supplementary-plane Unicode characters
on Windows" — this implies the cell-based renderer's Windows clipboard integration was
actively broken during development and required targeted fixes.

The 1.0.55 CJK fix addresses a specific subset of clipboard failures (non-ASCII on
Windows) but the broader copy flow for ASCII/Latin content appears to remain broken.

**Why the paste fixes in 1.0.56–1.0.57 do not address this:**
Multiple clipboard-related fixes landed in 1.0.56 and 1.0.57 (tab cursor fix, empty
line fix, Wayland fix), but all of these address pasting *into* the prompt, not
copying *from* the response.

**Workaround:** Revert to **1.0.48** (or possibly anything prior to 1.0.55 — versions
1.0.49–1.0.54 not tested).

**Environment:**
- OS: Windows 11 (Build 26100)
- CPU: x86_64
- Terminal: Cygwin (mintty), cmd.exe in conhost (Start menu) — reproduced in both
- Shell: bash (Cygwin) / cmd
- Node.js: v24.15.0
- Installation: `npm install -g @github/copilot`

**Related issue:** #3621 (auto-compaction loop) — both bugs manifest simultaneously and
both regressed in the 1.0.52–1.0.55 timeframe.

Beginne damit, den Kopierfehler unter Windows im zellenbasierten Terminal-Renderer zu reproduzieren, und vergleiche dabei die Versionen 1.0.48 und 1.0.55 oder höher. Verfolge die Windows-Integration der Zwischenablage im Renderer und überprüfe, dass die ausgewählte Ausgabe die vorhandene Zwischenablage sowohl in mintty als auch in conhost ersetzt, ohne die Unicode-Verarbeitung zu beeinträchtigen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.