fix(build): always produce linux/amd64 agentctl as part of \make build\

Remote executors (local Docker, remote Docker, Sprites) bind-mount the
linux/amd64 agentctl binary into the container. On non-Linux hosts (e.g.
macOS dev installs) running plain \`make build\` produced only the host
binary, so the first Docker session would fail with:

  failed to build container config: agentctl linux binary not found

The top-level \`make dev\` already special-cased non-Linux hosts to chain
\`build-agentctl-linux\`, but \`make build\` and \`make start\` skipped it.
Add it as a dependency of the standard backend build so every install
ships a usable linux variant. Cross-compiling agentctl is fast (~1-2s)
and the binary is ~20 MB, so the cost is negligible.

Users who hit the error can run \`make build-backend\` (or \`make build\`
in apps/backend) to regenerate; setting KANDEV_AGENTCTL_LINUX_BINARY
remains a manual override.

apps/backend/Makefile

@@ -52,8 +52,11 @@ build-acpdbg:
 	@$(MKDIR) $(BUILD_DIR)
 	$(GO) build $(GOFLAGS) -ldflags "$(LDFLAGS)" -o $(BUILD_DIR)/acpdbg ./cmd/acpdbg
 
-## Build the unified kandev binary
-build: build-agentctl build-mock-agent build-acpdbg
+## Build the unified kandev binary. Always includes the linux/amd64 agentctl
+## variant because remote executors (local Docker, remote Docker, Sprites)
+## bind-mount it into containers — without it, the first Docker session on a
+## non-Linux host fails with "agentctl linux binary not found".
+build: build-agentctl build-agentctl-linux build-mock-agent build-acpdbg
 	@echo "Building $(BINARY_NAME)..."
 	@$(MKDIR) $(BUILD_DIR)
 	CGO_ENABLED=1 $(GO) build $(GOFLAGS) -ldflags "$(LDFLAGS)" -o $(BUILD_DIR)/$(BINARY_NAME) ./cmd/kandev