debug-busybox: pass Copilot prompt via @file to avoid Windows

8 KB argv limit

The first run at
https://github.com/git-for-windows/MINGW-packages/actions/runs/26983788763/job/79628887416
hit "The command line is too long." instantly when invoking
copilot.cmd because the rendered prompt is ~15 KB and the
copilot.cmd npm shim funnels its non-interactive prompt through
cmd.exe, whose per-command limit is around 8 KB on Windows. The
shim turned the prompt into a single argv string and CreateProcess
refused before Copilot ever started.

Copilot CLI's @path syntax dereferences a file at prompt-evaluation
time and inlines its contents as context rather than transporting
them over argv, so a short literal "Follow the instructions in
@copilot-prompt.md exactly." crosses the cmd.exe boundary cleanly
and the actual instructions go in through the same channel Copilot
already uses for codebase context. This works identically on
Linux and macOS where the limit would not have hit, so no
per-platform branching is needed.

The substitution from envsubst (the previous step that produces
copilot-prompt.md from the template) is preserved unchanged; only
the way Copilot ingests that rendered file is different.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/debug-busybox.yml

@@ -117,8 +117,13 @@ jobs:
           MSYS2_PATH_TYPE: inherit
           CHERE_INVOKING: 1
         run: |
-          prompt="$(cat copilot-prompt.md)" &&
-          copilot.cmd -p "$prompt" --yolo </dev/null
+          # Reference the rendered prompt with `@path` instead of
+          # `-p "$(cat ...)"`: copilot.cmd funnels its argv through
+          # cmd.exe whose 8 KB-per-command limit this prompt blows
+          # past, producing "The command line is too long." before
+          # Copilot ever starts.
+          copilot.cmd -p "Follow the instructions in @copilot-prompt.md exactly." \
+            --yolo </dev/null
 
       - name: Archive Copilot session state
         if: always()