Document global debug.json usage and fix REPL error copy (#40836)

## Overview
- document how to keep a per-user debug.json so global launch tasks show
up everywhere (Fixes #39849)
- sanitize REPL terminal text before copying so error blocks can be
copied and opened in buffers (Fixes #40207)

## Design Decisions
- reused the existing user debug file (paths::debug_scenarios_file) and
pointed docs at the zed::OpenDebugTasks command to stay aligned with the
settings UX
- extract a sanitize helper inside TerminalOutput::full_text to strip
\r/null padding while keeping indentation intact, then join the cleaned
lines so clipboard and buffers get readable text

## Testing
- Not run (cargo is unavailable in this environment)

Fixes #39849.
Fixes #40207.

docs/src/debugger.md

@@ -56,6 +56,16 @@ Check the documentation for your language for example configurations covering ty
 
 Zed will also load debug configurations from `.vscode/launch.json`, and show them in the new process modal if no configurations are found in `.zed/debug.json`.
 
+#### Global debug configurations
+
+If you run the same launch profiles across multiple projects, you can store them once in your user configuration. Invoke {#action zed::OpenDebugTasks} from the command palette to open the global `debug.json` file; Zed creates it next to your user `settings.json` and keeps it in sync with the debugger UI. The file lives at:
+
+- **macOS:** `~/Library/Application Support/Zed/debug.json`
+- **Linux/BSD:** `$XDG_CONFIG_HOME/zed/debug.json` (falls back to `~/.config/zed/debug.json`)
+- **Windows:** `%APPDATA%\Zed\debug.json`
+
+Populate this file with the same array of objects you would place in `.zed/debug.json`. Any scenarios defined there are merged into every workspace, so your favorite launch presets appear automatically in the "New Debug Session" dialog.
+
 ### Launching & Attaching
 
 Zed debugger offers two ways to debug your program; you can either _launch_ a new instance of your program or _attach_ to an existing process.