feat: remote-build shim via sand-boxer (SAND-WP-0012)

Delegate make remote-build to sandboxer create when CLI is available;
retain legacy rsync path with deprecation notice. Add verify script.

infra/build-machines/Makefile

@@ -15,11 +15,37 @@ sync:
 	  --exclude='*.o' --exclude='*.hi' \
 	  $(PROJECT)/ $(VM):$(RDIR)/
 
-# Run cabal build on VM after sync
+# Run cabal build on VM — prefers sand-boxer workspace (SAND-WP-0012 shim)
 .PHONY: remote-build
-remote-build: sync
+remote-build:
+	@if command -v sandboxer >/dev/null 2>&1; then \
+	  $(MAKE) remote-build-sandboxer; \
+	else \
+	  echo "WARN: sandboxer not on PATH — using legacy rsync-only path" >&2; \
+	  $(MAKE) remote-build-legacy; \
+	fi
+
+# Legacy rsync + ssh (deprecated — install sand-boxer for isolated workspaces)
+.PHONY: remote-build-legacy
+remote-build-legacy: sync
 	ssh $(VM) "cd $(RDIR) && source ~/.ghcup/env && cabal build all 2>&1"
 
+.PHONY: remote-build-sandboxer
+remote-build-sandboxer:
+	@set -euo pipefail; \
+	STATUS=$$(sandboxer create \
+	  --profile profile.vm-haskell-build \
+	  --input vm=$(VM) \
+	  --input repo=$(PROJECT) \
+	  --actor agt \
+	  --project the-custodian \
+	  --host localhost); \
+	ID=$$(echo "$$STATUS" | python3 -c "import sys,json; print(json.load(sys.stdin)['sandbox_id'])"); \
+	RDIR=$$(echo "$$STATUS" | python3 -c "import sys,json; r=json.load(sys.stdin).get('reachability') or {}; print(r.get('remote_dir',''))"); \
+	test -n "$$RDIR"; \
+	ssh $(VM) "cd $$RDIR && source ~/.ghcup/env && cabal build all 2>&1"; \
+	sandboxer destroy "$$ID"
+
 # Run tests on VM
 .PHONY: remote-test
 remote-test: sync

infra/build-machines/README.md

@@ -75,7 +75,7 @@ ssh haskell-build          # should connect via tunnel
 ## Using the VM
 
 ```bash
-# Build a Haskell project remotely
+# Build a Haskell project remotely (prefers sand-boxer workspace when installed)
 make remote-build PROJECT=~/projects/my-app
 
 # Run tests