AGENTS.md: note DOTNET_CLI_HOME workaround for sandboxes that block ~/.dotnet-cli

.gitignore

@@ -2,4 +2,5 @@
 artifacts/
 bin/
 obj/
+.dotnet-cli/
 *.user

AGENTS.md

@@ -21,6 +21,12 @@ The wrappers cd to the repo root (the test runner must run from the parent
 directory of the test project), export `DOTNET_CLI_HOME`, and invoke
 `mise exec -- dotnet ...`. Do not call bare `dotnet`.
 
+If the first-time-use setup fails with `The file ...
+'.dotnet-cli/.dotnet' already exists` or `Operation not permitted` (some
+agent/sandbox shells block `$HOME/.dotnet-cli`), point the wrapper at a
+repo-local home: `DOTNET_CLI_HOME="$PWD/.dotnet-cli" scripts/dotnet.sh ...`
+(add `.dotnet-cli/` to `.gitignore`).
+
 ## Yaks (task management with yx)
 
 Tasks are managed with the `yx` CLI; yaks live in git (event store), snapshot